[SettingsProvider] minor refactor: use a separate lock for GenerationRegistry

This is a preparation to introduce ReadWrite lock for SettingsProvider.

This is a no-op for now because both addGenerationData and incrementGeneration are called under SettingsProvider's mLock.

BUG: 284452689
Test: presubmit

Change-Id: I0fcb2ff11d3d2277da5e96f1409a57c76139a959
3 files changed