Use tip-of-tree robolectric

By adopting robolectric built-from-source, the backup tests are no
longer grafting ToT backup classes on top of SDK 26 framework classes.

Instead, all tests are executed against the SDK built from current
master which includes the backup classes under test.

Bug: 117904612
Test: make -j56 RunFrameworksServicesRoboTests
Change-Id: Iaf911db93f318df46e47b765f94701b36bacfe59