sourceCompatibility 1.7 and targetCompatibility 1.7 are obsolete, use 1.8 by default

[misl6]

While upgrading to the new SDL2 (in fact the CI is failing), which uses lambdas in Java, I noticed we still have set both sourceCompatibility and targetCompatibility to 1.7 by default, unless androidx is enabled.

This PR makes 1.8 the default, even if androidx is disabled.

Some additional info about Java API on Android (and support for old APIs):

• https://developer.android.com/studio/write/java8-support
• https://developer.android.com/studio/write/java8-support-table

From #2927 CI run:

> Task :compileDebugJavaWithJavac FAILED

warning: [options] source value 7 is obsolete and will be removed in a future release

warning: [options] target value 7 is obsolete and will be removed in a future release

warning: [options] To suppress warnings about obsolete options, use -Xlint:-options.

/Users/admin/.python-for-android/dists/bdist_unit_tests_app/src/main/java/org/libsdl/app/SDLAudioManager.java:37: error: lambda expressions are not supported in -source 7

                    Arrays.stream(addedDevices).forEach(deviceInfo -> addAudioDevice(deviceInfo.isSink(), deviceInfo.getId()));

                                                                   ^

  (use -source 8 or higher to enable lambda expressions)

/Users/admin/.python-for-android/dists/bdist_unit_tests_app/src/main/java/org/libsdl/app/SDLAudioManager.java:329: error: method references are not supported in -source 7

            return Arrays.stream(audioManager.getDevices(AudioManager.GET_DEVICES_OUTPUTS)).mapToInt(AudioDeviceInfo::getId).toArray();

                                                                                                                      ^

  (use -source 8 or higher to enable method references)

2 errors

3 warnings

[AndreMiras]

LGTM