Adds interface to show magnification button for System UI

This change just add the interface and the method.
We'll apply this change in the later change.
So that magnification button UI will be updated by
showMagnificationButton() when magnification mode or
capability settings changed.

Bug: 145780606
Test: atest WindowMagnificationConnectionWrapperTest WindowMagnificationManagerTest
Change-Id: I544b03f62a0a67318585620db61caaa6dc213206