SDK / exoplayer

With this change, it becomes possible to depend on ExoPlayer
locally in settings.gradle by doing:

gradle.ext.exoplayerRoot = 'path/to/exoplayer/root'
apply from: new File(gradle.ext.exoplayerRoot, 'core_settings.gradle')

You can optionally add a prefix onto ExoPlayer's module names
by adding:

gradle.ext.exoplayerModulePrefix = 'prefix'

Issue: #2851
Issue: #2974

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=160277967