Support screenshot override (Robolectric local runs).

Bug: 336590826
Test: atest -c SystemUIGoogleRoboRNGTests -- --test-arg \
    com.android.tradefed.testtype.IsolatedHostTest:java-flags:-Dscreenshot.writeScreenshotToLocal=yes; \
    atest -c SystemUIGoogleRoboRNGTests
    (Before the first run, we can manipulate the Robo screenshots
    arbitrarily. The second run is expected to pass)
Change-Id: I853ee7a0702247b162a38fcee87d566ac4b975a1
2 files changed