SDK / exoplayer

The blaze BUILD file for the Cronet extension now has three options:
Using native bundled Cronet libs, using GMSCore, or using whichever is newer.
The GMSCore version is preselected (as it is the smallest), but other variants
may be used by uncommenting the respective lines.

The API is the same for all cases and the CronetEngine.Builder automatically
selects the newest option or falls back to default http.

To avoid that apps using this extension need to add a dependency to Cronet
themselves, I added a CronetEngineFactory to the Exoplayer extension.

Gradle builds can't be supported (as far as I can see), as the GMSCore Cronet
version is first party only.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=154812029