DO NOT MERGE Fix crash related to getting the root of an unattached view

view.getRootView returns itself when unattached, which may not be a
ViewGroup instance. Therefore trying to cast to a ViewGroup will cause a
ClassCastException. This fix avoids the exception by ensuring that the
view is attached before trying to cast the root view.

Fixes: 277637573
Test: mp droid and verified touch gestures on dream work correctly by
swiping up the bouncer, and swiping down the notification shade.

Change-Id: Ic2d87278e5c60509c39062437b548ae76959b169