Use ReentrantReadWriteLock in the compat framework

The compat config is read frequently but modified infrequently (i.e.
only when new overrides are added), so use a ReentrantReadWriteLock
instead of the default reentrant lock.
Test: atest CompatConfig
Bug: 191384882
Bug: 191718541

Change-Id: I21da41ea51bb722c45245eccb02652b375be9f12