Use taskbarSize to calculate clippedInsets

- Instead of calculating clippedInsets from thumbnail dimension, always use taskbarSize
- Clean up clippedInsets code as only bottom inset is ever present

Fix: 235328056
Test: With taskbar hidden, out of orientation thumbnail has proper size in quickswitch and overview
Change-Id: I3d7fc7899f04862c3401935b10f475e9ab74685a