HFP: HeadsetStateMachine logging clean-up

* Enable logging at critical code path to indicate connection status
  more clearly
* Disable duplicate logging statements to reduce log spam
* Simplified method usages to reduce code size
* Remove unncessary null checks such as device null check in CONNECT
  message, we should crash if device is null in CONNECT message

Bug: 63775402
Test: HFP PTS test, HFP regression, no functional changes
Change-Id: I97d3e82e3e5cf3e4e2b702f6b3d8d054e0962272