Add swipe-to-dismiss support to PhoneWindow.

This adds a new window feature -- FEATURE_SWIPE_TO_DISMISS -- and a
theme attribute to activate that feature. When the feature is
activated, a SwipeDismissLayout is inflated as the DecorView layout.
SwipeDismissLayout intercepts touch events and steals ones that are
large swipes to the right if its children don't. PhoneWindow registers
handlers that listen for these swipe events, translate the window when
necessary, and finish the activity at the end of the gesture.

Change-Id: I512e758f3c3ffd3b353dba3b911c0e80a88d6f5f