Force AlertDialogLayout to have gravity start|top

AlertDialogLayout overrides LinearLayout#onMeasure but not onLayout,
meaning that some state initialized for the handling of gravity may
not be valid in LinearLayout#onLayout. As this is an internal class
never used directly by apps, explicitly specify the default gravity of
start|top in @layout/alert_dialog_material to avoid this bug. Apps
that do things like set gravity in their theme (for whatever reason)
could otherwise change its behavior.

Bug 30494039

Change-Id: I71a8be1829a7fe24cf7714a3bd5ed732f85eb887
(cherry picked from commit 39e0bf23)