Make snapshot surface relative to parent

- When in splitscreen and the snapshot aspect ratio matches the parent
  we need to offset to ensure that the calculated matrix is relative
  to the parent otherwise

Change-Id: I2fcd72bde40171fb456108993214b00741abe7fd
Bug: 157624680
Test: Enable waterfall/tall cutouts, quickswitch in both full and
      splitscreen
Test: atest TaskSnapshotSurfaceTest