Add ScreenshotClient functionality for returning GraphicBuffer.

WM and SysUI are interested in screenshotting to GraphicBuffer, for
zero copy initialization of Starting Windows and usage in SysUI. Both
codepaths are co-existing in the WM now, so existing screenshot APIs
are not cleaned up, though perhaps they could be soon.

Test: Manual in combination with other branches.
Bug: 31339431
Change-Id: I11151f44eddbba53559befebf5ee2bde1dee9b40