Introduced AccountDb API replacing helpers

Refactored AccountsDb to provide unified interface for interacting with
CE/DE database.

Benefits of the new API:
- Better encapsulation - only relevant methods are exposed.
  Implementation details are hidden in private helper classes.
- Clients can now treat CE tables as if they were in  DE database, but
  only available when the user is unlocked.
- Opening database is now implicit - no more getReadableDatabase
  /getWritableDatabase calls. Clients only need to define transaction
  boundaries, when necessary.

Test: Refactoring
Bug: 30639520
Change-Id: I08b1fc95b7a633b036e05e8ef7e8ebd239187aa1