SDK / exoplayer

This ensures we have full test coverage for proguard configs now.
The only configs not covered by tests are:
 - IMA and OkHttp which copy recommended configs from the respective
   library. I couldn't reproduce failures by removing them (and thus
   couldn't write a test that ensures they are correct).
 - Some dontwarn lines that just suppress warnings.

In addition, this change fixes a couple of related issues:
 - Moved AV1 proguard config to correct module.
 - Removed mentioning of deprecated ExtractorMediaSource from README
 - Suppressed warning from IMA code that prevent proguarding under
   strict rules
 - Fixed wrong proguard exclusion in VP9 module.
 - Moved FLAC exclusion (DefaultExtractorsFactory) to correct module.
 - Added AlertDialog suppression for de-jetified code.
 - Removed unusued dependency from UI module that causes large APK
   size increase.

#exofixit
#minor-release

PiperOrigin-RevId: 344427532