MetadataRetriever: Use RGBA_8888 as default color format based on a flag

When the flag mediametadataretriever_default_rgba8888 is set, use
RGBA_8888 as the color format in getFrameAtTime call.

Add the unit test to check the intended behaviour.

Test: adb shell setprop persist.device_config.aconfig_flags.\
media_solutions.com.android.media.playback.flags.\
mediametadataretriever_default_rgba8888 true
Test: adb shell device_config put media_solutions \
com.android.media.playback.flags.mediametadataretriever_default_rgba8888 \
true
Test: atest com.android.mediaframeworktest.unit.MediaMetadataRetrieverTest
Bug: 298965955

Change-Id: I8452e75e07bcfcd2483ad2da971806147f5a9067