Floating toolbars: Encapsulate StandaloneActionMode view creation.

This CL defines a new interface to be used by ActionModeWrapper.
This allows each client to inject a different primary ActionMode to
the wrapper and keep view creation code next to ActionMode
creation.
The interface method is only called when the wrapper actually
knows that will be the used type, avoinding unnecessary view
creations.

Things pending after this CL:
- Correct handling of ActionModes created by
callback.onWindowStartingActionMode(). This includes all current usages
in an existing ActionBar, as it is handled by Activity. In the current
state, we do not intercept these ActionModes and hence cannot change the
representation.
- Representing the floating type
- Supporting two ActionModes in parallel in DecorView, one of each type

Change-Id: Ic38e209877c3876161d8dd56902e25b51fbe40b6