Fix floating toolbar touchable region bug.

The floating toolbar is rendered inside a transparent popup window
so that we can have smooth animations inside of the window (NOTE that
animating a view and a window at the same time do not work well as
both are not in sync.) Having a transparent window implies that we
can't get touch events through to the content below the window for
areas covered by the transparent window. To handle this issue we
"burn a hole" through the window by specifying it's "touchable area".
This is done using the mInsetsComputer which is set on the popup
window's root view's tree observer. When the popup is dismissed and
re-shown, we need to reset the mInsetsComputer as we most likely
won't have that root view any longer.

Change-Id: I13ea50bc19950180d339d05d82cbee03230e2f05