Perform all start/stop A2dpService operations in start() and stop() methods

* The A2dpService.start() and stop() methods now contain all
  start/stop operations.
* The order of the operations in stop() is reverse of the order of
  the operations in start()
* Remove A2dpService() default class constructor: it is empty and not
  necessary.

Test: Manual: (re)start A2DP, connect/disconnect to a headset
      Unitests: runtest bluetooth
Bug: 72948646
Bug: 73205770
Change-Id: I4ed0fb24a972140cc82aaf465d8ec1142ab02722