PopupWindow: Be more consistent about Gravity calculation.

Currently we are only appending the DISPLAY_CLIP_VERTICAL
if the API user happens to call update() after showAsDropDown().
This fixes that behavior, and fixes a RTL issue (START v. LEFT).
Unblocks a CTS test I am working on.

Bug: 30578621
Change-Id: Ibf838544d453986b66eb223e2ac0f7d909fe5630
(cherry picked from commit 8367c509)