PopupWindow: Gravity fixes.

First we restore the M semantics with respect
to DISPLAY_CLIP_VERTICAL, which we only applied
for drop downs, and omitted in the case of
showAtLocation. Further, we fix an error where
user specified gravity from showAtLocation is
erased when calling update() by storing the
gravity and including it in computeGravity().

Bug: 30445010
Bug: 30965176
Change-Id: I28a081e1237a8b41f2444717e0db21ef4181507b