Convert screenshot hw Bitmap to sw Bitmap for screenshot preview.

Bitmaps created from SurfaceControl.screenshot are hardware Bitmaps. In
order to update the size for the screenshot preview, the Bitmaps need to
be converted to software Bitmaps first.

Also updated the SurfaceControl docs to make this clear.

Fixes: 69929720
Test: Notifications from lock screen and status bar show the screenshot
preview.

Change-Id: Iebc4a261216f3d965e1c98a77c27cc0cb2e442af