[xabuild] Add MSBuild wrapper script to build Xamarin.Android projects

(IDEs?! We don't need no stinkin' IDEs! [0, 1])

`tools/scripts/xabuild` is an `xbuild` ("MSBuild") -using script to
support building Xamarin.Android projects from the command-line,
without modifying or replacing the "system" Xamarin.Android install.
Also handy if there are multiple development branches available.

`tools/scripts/xabuild` takes the same command-line arguments as
xbuild(1) and MSBuild, allowing projects to be built and application
packages to be created:

  ${xamarin-android-checkout-path}/tools/scripts/xabuild /t:SignAndroidPackage
  adb install bin/Debug/*-Signed.apk

Xamarin.Android.Build.Tasks.dll is updated to look for an alternate,
*available*, path to `libmonosgen-2.0.so`, and to remove the
%(Reference.Private) metadata so that all required assemblies are
installed into $(OutputPath). (Previous versions of Xamarin.Android
would encode the ABI into the filename, but this encoding should no
longer be necessary.)

android-toolchain is altered to provide `_GetAndroidNdkDirectory` and
`_GetAndroidNdkDirectory` targets, so that the xabuild script can
properly determine the path to the $(AndroidToolchainDirectory)
location.

[0]: In the spirit of: https://www.youtube.com/watch?v=VqomZQMZQCQ
[1]: Seriously, we often like to keep the "system" Xamarin.Android
    untouched and unchanged, so that we can more easily build filed
    bugs/etc., so the easiest way to build a Xamarin.Android project
    from a development branch is from an `xbuild` wrapper script and
    building from the command line [2].
[2]: https://developer.xamarin.com/guides/android/under_the_hood/build_process/#Build_Targets

build-tools/android-toolchain/android-toolchain.mdproj

@@ -6,7 +6,7 @@
     <ItemType>GenericProject</ItemType>
     <ProjectGuid>{8FF78EB6-6FC8-46A7-8A15-EBBA9045C5FA}</ProjectGuid>
   </PropertyGroup>
-  <Import Project="..\..\Configuration.props" />
+  <Import Project="android-toolchain.targets" />
   <PropertyGroup Condition=" '$(Configuration)|$(Platform)' == 'Debug|AnyCPU' ">
     <OutputPath>$(AndroidToolchainDirectory)</OutputPath>
   </PropertyGroup>
@@ -23,7 +23,6 @@
 			_CreateNdkToolchains;
     </BuildDependsOn>
   </PropertyGroup>
-  <Import Project="android-toolchain.targets" />
   <ItemGroup>
     <ProjectReference Include="..\..\src\Xamarin.Android.Tools.BootstrapTasks\Xamarin.Android.Tools.BootstrapTasks.csproj">
       <Project>{E8492EFB-D14A-4F32-AA28-88848322ECEA}</Project>

build-tools/android-toolchain/android-toolchain.targets

@@ -1,5 +1,6 @@
 <?xml version="1.0" encoding="utf-8"?>
 <Project DefaultTargets="Build" ToolsVersion="4.0" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
+  <Import Project="..\..\Configuration.props" />
   <Import Project="android-toolchain.projitems" />
   <Target Name="_CopyBootstrapTasksAssembly"
       Outputs="$(OutputPath)\Xamarin.Android.Tools.BootstrapTasks.dll">
@@ -85,4 +86,16 @@
         AlwaysCreate="True"
     />
   </Target>
+  <Target Name="_GetAndroidSdkDirectory">
+    <Message
+        Text="$(AndroidToolchainDirectory)\sdk"
+        Importance="High"
+    />
+  </Target>
+  <Target Name="_GetAndroidNdkDirectory">
+    <Message
+        Text="$(AndroidToolchainDirectory)\ndk"
+        Importance="High"
+    />
+  </Target>
 </Project>

src/Xamarin.Android.Build.Tasks/Tasks/BuildApk.cs

@@ -425,7 +425,7 @@ void AddRuntimeLibraries (ZipFile apk, string supportedAbis)
 				apk.AddEntry (string.Format ("lib/{0}/libmonodroid.so", abi), File.OpenRead (path));
 				if (!use_shared_runtime) {
 					// include the sgen
-					library = string.Format ("libmonosgen-2.0.{0}.so", abi);
+					library = "libmonosgen-2.0.so";
 					path = Path.Combine (root, "lib", abi, library);
 					apk.AddEntry (string.Format ("lib/{0}/libmonosgen-2.0.so", abi), File.OpenRead (path));
 				}

src/Xamarin.Android.Build.Tasks/Xamarin.Android.Build.Tasks.csproj

@@ -59,11 +59,9 @@
     </Reference>
     <Reference Include="Ionic.Zip">
       <HintPath>..\..\packages\Unofficial.Ionic.Zip.1.9.1.8\lib\Ionic.Zip.dll</HintPath>
-      <Private>False</Private>
     </Reference>
     <Reference Include="FSharp.Compiler.CodeDom">
       <HintPath>..\..\packages\FSharp.Compiler.CodeDom.1.0.0.1\lib\net40\FSharp.Compiler.CodeDom.dll</HintPath>
-      <Private>False</Private>
     </Reference>
   </ItemGroup>
   <ItemGroup>

tools/scripts/xabuild

@@ -0,0 +1,25 @@
+#!/bin/bash -e
+topdir="$(cd `dirname "$0"`/../.. && pwd)"
+
+if [ -z "$CONFIGURATION" ] ; then
+	CONFIGURATION=Debug
+fi
+
+export TARGETS_DIR="$topdir/bin/$CONFIGURATION/lib/xbuild"
+export MSBuildExtensionsPath="$TARGETS_DIR" 
+export MONO_ANDROID_PATH="$topdir/bin/$CONFIGURATION"
+export XBUILD_FRAMEWORK_FOLDERS_PATH="$topdir/bin/$CONFIGURATION/lib/xbuild-frameworks"
+ANDROID_NDK_PATH=$(cd $topdir/build-tools/android-toolchain && xbuild /nologo /v:minimal /t:_GetAndroidNdkDirectory android-toolchain.targets)
+ANDROID_SDK_PATH=$(cd $topdir/build-tools/android-toolchain && xbuild /nologo /v:minimal /t:_GetAndroidSdkDirectory android-toolchain.targets)
+
+ANDROID_NDK_PATH=$(echo $ANDROID_NDK_PATH | sed 's/^\w*//g')
+ANDROID_SDK_PATH=$(echo $ANDROID_SDK_PATH | sed 's/^\w*//g')
+
+export ANDROID_NDK_PATH
+export ANDROID_SDK_PATH
+
+exec xbuild $MSBUILD_OPTIONS \
+	/p:AndroidNdkDirectory="$ANDROID_NDK_PATH" \
+	/p:AndroidSdkDirectory="$ANDROID_SDK_PATH" \
+	/p:MonoDroidInstallDirectory="$MONO_ANDROID_PATH" \
+	"$@"