Add proguard rule for error prone annotations.

[danh32]

Was seeing proguard build failures due to the @DoNotMock annotation.

This change adds an additional proguard rule to exclude the compileOnly errorprone annotations for all Android modules.

Tested by running a proguard build of Robinhood with the given rule.

[CLAassistant]

All committers have signed the CLA.

[ZacSweers]

Thanks! I'll cut a patch release with these soon

[danh32]

Thanks! No rush, happy to wait for the next scheduled release if nothing else big is pending.