New hearing aid info side "SIDE_MONO"

Monaural hearing aid might set its audio location as
AUDIO_LOCATION_MONO, so we wouldn't know which side of the device is.
Create a new side of SIDE_MONO for this kind of devices and return the
corresponsing summary string according to it.

Flag: EXEMPT bugfix
Bug: 379616650
Test: atest HearingAidDeviceManagerTest
Test: atest CachedBluetoothDeviceTest
Test: manually check the UI with real device
Change-Id: Ia09298cfb9a869f40f22ec93ae075a412af09f57