Introduce communal state proto for backup & restore

- Add a communal_hub_state proto buffer that represents the state of the
  database, used for backup & restore
- Add a utility that takes a snapshot of the database and converts it
  to a state protobuf
- Add a DAO transaction that takes a snapshot of the database and
  returns a state proto
- Make CommunalDatabase a singleton for access outside of the dagger
  dependency graph

Test: atest CommunalBackupUtilsTest
Bug: 309809222
Flag: ACONFIG com.android.systemui.communal_hub TEAMFOOD
Change-Id: Id43281e198f9e56f7bcfb930d47b58ca4571098e