Moved the scrolling logic away from the onDraw

Having the scrolling animation in onDraw and directly
Updating the notifications there could lead to multiple
issues:
1. Certain state might not be up to date since a layout
Was still pending
2. The logic for drawAsSrc could lead to schmutz on the
Screen since the scrim view has already drawn.

Test: add notifications scroll
Change-Id: Id9ae562c1e4b353d269de755f491900a6073c5d8
Fixes: 34631572