Add gravity settings to PopupWindow/ListPopupWindow/PopupMenu

Allow calling code to specify left/right/start/end gravity when
showing a popup attached to an anchor. This allows easy alignment of
either the right or left edges of the popup and anchor view.

Bug 10728401

Change-Id: Ie0844a04ea0576fa67b0972f5873aaa4c5b823f6
6 files changed