Make Scenes and Transitions first-class in PhoneWindow/Themes

Add a window feature for content transitions. This implicitly creates
a Scene for each setContentView operation and runs the appropriate
transition. Applications can specify a TransitionManager XML in their
theme that will apply the appropriate transitions when these implicit
scene changes occur. Apps can specify a "to" with no "from" in a
transition to request an entrance transition for the given
content. This lays the groundwork for further full content
change/activity to activity transitions.

Change-Id: Ic815d9e0b9ce958152d70bf6ee01be075aa9fe88