Simplify app-locales B&R stage data retention.

Store the stage data in memory only (not in file anymore). Delete it
after min(retention_period, next_reboot). Also check if we need to
remove the stage data when doing lazy restore. Modify the unit tests
accordingly.

Bug: 210869496
Test: atest FrameworksServicesTests:LocaleManagerBackupRestoreTest
Change-Id: Ib5be97cede7c340541cab2f0dffad0a1c2d20f87