Remove SystemUI ScreenshotTestRule (1/2)

This CL removes SystemUI ScreenshotTestRule now that most of what it
does was contributed back to the platform screenshot library in
ag/19135707.

Bug: 230832101
Test: atest SystemUIGoogleScreenshotTests
Change-Id: I38d01bcd35e6263752c69539dd65950d898f53de
Merged-In: I38d01bcd35e6263752c69539dd65950d898f53de
8 files changed