HFP: Use objects for shared library interface and callbacks (1/2)

* Declare bluetooth::headset::Interface and
  bluetooth::headset::Callbacks instead of using structs
* Put HFP related methods and types into bluetooth::headset namespace
* Rename those methods into CamelStyle according to Google C++ style
  guide
* Remove the usage of HAL_CBACK in HFP as we only need to check if the
  callback object is null and all its methods must be implemented as
  mandated by rules of the pure virtual function
* These classes can be mocked during test to isolate behaviors
* Improved effort handling in btif_hf.cc to log error messages during
  failures

Bug: 70538124
Test: unit tests, testplans/135585
Change-Id: Iba120ef9d0a9701aececd300395b7b18d2d44e7d