postAtFrontOfQueue() in ICompanionDeviceService.Stub

postAtFrontOfQueue() instead of post()-ing at the back of the queue
in ICompanionDeviceService.Stub to prevent "posted" methods are invoked
*before* onUnbind()/onDestroy().

Add missing @Override annotation to onDispatchMessage method.

Clean up dispatchMessage() which is not a part of
ICompanionDeviceService interface.

Make the Stub implementation class private.

Bug: 198485833
Bug: 200940347
Test: atest CtsCompanionDeviceManagerCoreTestCases
Test: atest CtsCompanionDeviceManagerUiAutomationTestCases
Test: atest CtsOsTestCases:CompanionDeviceManagerTest
Change-Id: Ic44e7968b1cef42355631151aa6958fe1ec7dd6e