R8 and byte-level optimizations should be enabled for release builds

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Vision statement / What / Requirements

All Mozilla Android products should use R8 and byte-level ProGuard optimizations.

Impact

Switching to R8 will significantly improve release build times. https://android-developers.googleblog.com/2018/11/r8-new-code-shrinker-from-google-is.html

Enabling byte-level optimizations will improve application run times.

Acceptance criteria

Both R8 and byte-level optimizations are enabled in release builds.