AdapterService: Do not call System.exit() on onUnbind

* Process termination should be handled by the OS
* System.exit() in onUnbind also prevents onDestroy() to be called
* Removed isMock() since its only use case is to check if System.exit()
  should be called

Bug: 35366908
Test: make, run java test, turning Bluetooth ON/OFF
Change-Id: Ieba927ce86d5dd27269f10188c74308fa76e66d9