Initialize static service reference properly

* Change I10ea7dabcb2e0d296384d6f9a60683569bdef3f8 changes the meaning
  of isAvailable() in ProfileService. As result, many service's static
  instance references no longer get assigned properly
* This CL fixes those issue for:
    A2dpSinkService
    AvrcpControllerService
    HeadsetClientService
    HidHostService
    MapClientService
    PanService
    PbapClientService
* This CL also adds a number of tests to start and stop each
  ProfileService and do some profile specific verification such as
  verifying HeadsetClientService.getHeadsetClientService()'s return
  result is not null
* Adds a TestUtils class to include common test routines such as:
  setAdapterService(), clearAdapterService(), startService(),
  stopService()
* Remove setting to null statements in tearDown() method as JUnit4
  runner will nullify fields by default

Fixes: 72816382
Test: 1) enable all profiles in config.xml 2) runtest -j32 bluetooth
Change-Id: If39c44097d5cfbf0d606dda20e5b8cdb565b5764