Fix test issues with media3 buttons

- Disable tests that are redundant or irrelevant with new buttons
- Replace assert causing some tests to crash
- Use mock Handler to avoid some tests hanging

Flag: com.android.systemui.media_controls_button_media3
Bug: 360196209
Bug: 377604849, 377710586
Test: MediaDataLoaderTest LegacyMediaDataManagerImplTest MediaDataProcessorTest Media3ActionFactoryTest
Change-Id: I9bad4deaf66081a242ca757b5a17e0178bc813ac