Expose SurfaceControl method to screenshot to GraphicBuffer.

A graphic buffer is most useful, as we can both attach it
to starting windows, and directly use it in Sys-UI. The old
codepath for starting windows/saved surfaces, is co-existing
at the moment, so I don't make large attempts to clean up
the existing screenshot code.

Bug: 31339431
Test: Manual test in combination with other branches
Change-Id: I562fdd5460dbce3201ba090272e8731850780f20