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 8367c50972)

core/java/android/widget/PopupWindow.java

@@ -1547,7 +1547,7 @@ public class PopupWindow {
         }
 
         // Let the window manager know to align the top to y.
-        outParams.gravity = Gravity.LEFT | Gravity.TOP;
+        outParams.gravity = computeGravity();
         outParams.width = width;
         outParams.height = height;