HFP: Use objects for shared library interface and callbacks (2/2)

* Declare bluetooth::headset::Interface and
  bluetooth::headset::Callbacks instead of using structs
* Rename methods into CamelStyle according to Google C++ style guide
* These classes can be mocked during test to isolate behaviors
* Replaced NULL with nulltpr
* Added more logging for connection and audio state callback in JNI

Bug: 70538124
Test: unit tests, testplans/135585
Change-Id: Ic62f143a5cd8c0f3f459d109d10b692a00d7a5af