Replace BluetoothHeadsetClientCall with internal HfpClientCall

HFP Client Call metadata does not need to be shared outside the
Bluetooth stack. We're free to define our own parcelable object
internally and have it passed back to us.

This change removes BluetoothHeadsetClientCall in favor of our own
private call metadata objects, HfpClientCall.

Tag: #refactor
Bug: 206035301
Test: atest BluetoothInstrumentationTests
Change-Id: I21822e3cdc338878822b46ace6bb24568bc8008a