Default resolution settings [Backend]

Added new APIs to DisplayManager to:
1. set the user preferred display mode,
2. clear user preferred display mode and
3. wheteher user has chosen a display mode

These new settings are stored in Settings.Global.

Bug: 192332479
Test: atest DefaultDisplayModeTest
Test: atest LocalDisplayAdapterTest
Change-Id: I3a60e271c7ec23c84e74487e89b3e29ca91e8a40