Preserve non-floating state when entering pinned stack.

When transitioning between the fullscreen and pinned states
we often have a situation where we go from having a navigation and
status bar in the window to not. We'd like to use the source bounds
animation to crop these out rather than a sudden jump or scaling
but in order to do so we need to ensure they last until the end
of the animation. We track this state, and return the appropriate
value from isFloating. Furthermore, we add support to the bounds
animation to use the content frame as a source bounds when there
is no source bounds present, this means that we can crop out the
navigation and status bar so they will be invisible by the end of
the animation.

Bug: 37531386
Test: Manual
Change-Id: I72c549e3a3318534428d17b68ebee5832c32e6d7