[Bluetooth] Define bluetooth repo to make fetches in the background.

We've been talking about modernizing the bluetooth code for a bit, and
moving these calls to the background thread seems like the perfect time
to start `BluetoothRepository`.

Fixes: 271058380
Test: atest BluetoothControllerImplTest BluetoothRepositoryImplTest
Test: manual: connect and disconnect various bluetooth devices and
ensure QS tile and status bar are updated appropriately

Change-Id: Id58c52481cbc165ed28c42985edb2f28469dc5fd