Add a unit test testStopA2dpService()

Also, added helper method
  TestUtils.runOnLooperSync(Looper looper, Runnable action)
that can be used to run synchronously a runnable action on a looper thread.

Bug: 73832547
Test: runtest bluetooth -c com.android.bluetooth.a2dp.A2dpServiceTest \
          --test-method testStopA2dpService
Change-Id: I07ceb3bf3b80f841d3443613b7a6e839497dd447