Introduce SystemUIScreenshotLib for screenshot testing

This CL introduces the SystemUIScreenshotLib, and in particular the
(View)ScreenshotTestRule, to easily create a screenshot diff test of
a given View for multiple configurations (devices, theme and
orientation).

Some diff tests will be added in follow-up CLs to keep the size of
this one reasonable.

Test: atest SystemUIGoogleScreenshotTests
Bug: 230832101
Change-Id: I4762ed3d575d7a9f0d8ac6d06026efe1ed2a52be