Change screenshot test to use RNG

Test: atest -c SettingsScreenshotRNGTests
 -- --test-arg
 com.android.tradefed.testtype.IsolatedHostTest:java-flags:
 -Drobolectric.graphicsMode=NATIVE
Fixes: 314807258
Change-Id: I4fa533c19d0b1386a7a4649ed88e3814816c1899
7 files changed