Test coverage for new FBE functionality

Added testRemovedAccountSync, which removes account when the user is locked,
and verifies that syncDeCeAccountsLocked works as expected.

Added testPreNDatabaseMigration to test that pre-N database with existing
data is properly upgraded and migrated.

Bug: 26279712
Change-Id: I3846c626f616b1c4830965c9f0cb27fa32ce6f54