Test: Fix broken tests in HeadsetClientStateMachineTest

* Refactor HeadsetClientStateMachineTest to use looper from
  HandlerThread created in the test
* Refactor the test to use morden Mockito annotations and initializers
* Refactor the test so that bootstrap code is re-used among different
  tests

Bug: 68711696
Test: runtest -i -j40 bluetooth; adb shell am instrument -w -e timeout_msec 300000
      'com.android.bluetooth.tests/android.support.test.runner.AndroidJUnitRunner'

Change-Id: Ic3bafb96273abf065ebab2af6edf36b0ebf519b9