Fix robolectric test failure

Default UserManager caches instance across entire test suite, so 
test becomes flakey depending on the order they are run. Using a 
a ShadowUserManager without the cache ensures each test class gets
a UserManager with clean state.

Change-Id: Ia54f6a3259859add5a1e5d0101829497fb985ab1
Fix: 38393235
Test: make RunSettingsRoboTests