Move Activity multi-window event logic out of the public methods

Move the logic for dispatching multi-window mode change events to
fragments and the activity window out of the public API onFoo methods
meant to be overridden by apps. This prevents problems if an extending
class forgets to call super.onFoo in these methods and removes the
need for doing so.

Bug 28667205

Change-Id: Ibf4c543e22b95753e1e4812bbd3e81f7648de54d