Only create one bitmap to clip popup item corners

Previously we created a bitmap the size of the entire popup item and
drew a rounded rect in it to clip the corners. Now we create a bitmap
that is only the size of the rounded corner radius, and rotate and
translate it to all four corners to achieve the same clipping.

Besides the reduced memory of this approach, it also simplifies things
when we reduce the item's height (e.g. when the second to last
notification is dismissed), because we no longer have to recreate the
background bitmap to match the new size; the translation calculations
in dispatchDraw() automatically take the current size into account.

Bug: 35869307
Change-Id: I938bbc4bd87479580287426b4197516caf1a64d4