Crop the center when no source rect hint

When there is no source rect hint, forge a crop that matches the given
aspect ratio and centered, therefore we can use a consolidated animation
path and fix the issue with the round corner, which was inproperly
scaled in the past.

Flag: NONE bug fix
Bug: 298409662
Test: Test the overlay in both gesture and button navigation mode,\
      with both YouTube and Google Maps,\
      in both folded and unfolded mode.
Change-Id: I8ae48b999159a51eda2ff41c49cdb3c4e1541176