Move WindowControllerCallback methods into a delagate.

Adding @hide public methods to commonly subclassed classes like Activity
is never safe, as they can clash with existing methods in apps. Moving
@hide interface implementations into an inner class avoids the need to
add new @hide public methods in many cases.

Bug: 144361565
Test: Build, flash, boot device.
Change-Id: Ie8459c5a542e6ff4058d4ce783e27ebcf9c41dbb