Floating toolbars: Add content rect related methods to API

This CL adds the ActionMode.Callback2 abstract class and the rect
invalidate method needed to add the content rect API for Floating
Toolbars. It also extends the existing ActionModeCallbackWrapper in
DecorView to handle the case when ActionMode.Callback is provided
instead of Callback2, falling back to a default implementation.

Change-Id: Ia918ddfcfdf73d0e4cafd24c4a0573245d497cfe