HFP: Add support for Multi-HFP (1/2)

* Create one HeadsetStateMachine for each HFP/HSP device during the
following event:
  - CONNECT request sent from API
  - CONNECTION_STATE_CHANGED event from stack
* Use a HashMap to manage these state machines
* Removed synchronized label for methods in HeadsetService because state
  machines are lazy initialized instead of being created in the
  beginning
* Remove code in HeadsetStateMachine that used to deal with multiple
  headset devices. Instead, move these logic to HeadsetService
* System state update is sent to one device at a time from the Java
  layer and updates from HFP/HSP devices are synced among them
* SCO audio is restricted to one device at a time
* In-band ringtone is disabled when multiple devices are connected
* Audio paramters are device specific and are cached within each device's
  state machine. It is only set to the audio framework in AudioOn state
* Reject connection request when number of connections goes above limit
* Added unit tests for HeadsetService to test multi-device scenario
* Add HeadsetObjectsFactory to ease mocking during unit tests

Bug: 68952570
Test: runtest -j40 bluetooth, native unit tests, HFP regression for 1
      device, simple tests for multiple devices
Change-Id: Id22c5cbddfc4f9f95e35f035fcab6c4a5d797ebf