Limit notification scroll up to status bar

Fixes: 186916207
Test: open shade, scroll
      => notifications clipped right below status bar
      => scrim positioned right below status bar
Test: close shade from scrolled state
      => notification clipping and scrim position
         move up, in sync with decreasing expansion fraction

Change-Id: Ifc150444e68f0ce871e5b90ea08c05adf849ac7a