SDK / exoplayer

- Centralise compileSdkVersion, targetSdkVersion and
  buildToolsVersion in a single place for gradle.
- Bump compileSdkVersion and targetSdkVersion to 24.
- Bump com.android.tools.build version to re-enable
  instant start.
- Bump targetSdkVersion in manifests (needed for
  internal builds).
- Use standard expandable_list_item from Android in
  the demo app, since ours doesn't look right when
  targeting API level 24. We were also setting the
  theme on the wrong element in the manifest, so I'm
  removing that line.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=131929216