Add explicit dep on services.backup

android_robolectric_test is having some implicit deps removed
from it, so add in what we depend on directly.

Test: m RunFrameworksServicesRoboTests
Change-Id: Ice2abeb3f4d031cfe15ce919b750d6de59316c1f
Merged-In: Ice2abeb3f4d031cfe15ce919b750d6de59316c1f