AudioDeviceBroker/Inventory: fix misc error-prone warnings

Make lock objects final.
Implement hashCode() for classes with equals()
Don't use StringBuilder
Fix javadoc errors
Remove unused methods isSpeakerphoneRequested
  and sendIMsg
Log errors instead of empty exception catches
Inner class can be static.
Remove unused constants.
Remove unused parameter "key" in connected devices.

Bug: 341995844
Test: m -j RUN_ERROR_PRONE=true services
Change-Id: I4ec5133fae2e184cadf651469e7fcff3db66ea1d