Prepare tests for impending robolectric merge

Setting's ShadowUserManager.reset() was unnecessary and collided with a
newly introduced reset() method in its super class. It has been removed.

Test: make -j56 RunSettingsGoogleRoboTests &&
  make -j56 RunSettingsRoboTests &&
  make -j56 RunSettingsLibRoboTests

Change-Id: I63c50eca0de3581fd4f64f4862233446f5937d0c