Implmented the aidl interface for window magnification

Implemented WindowMagnificationConnectionImpl in WindowMagnification.
All requestes should post to main thread to change the UI.
This interface will be created when getting the request from
Accessibility Framework.

Bug: 146400227
Test: manual test
Change-Id: I0b6033dca3d53e7d271b84a5a3008dccf7447d71