Fix assertion in VisibilityPreferenceControllerTest.

The test was asserting that VISIBILIY_NO_OVERRIDE (an int)
is not contained in a List<String> (*), which is (trivially)
true but doesn't test what the test intends to test.

(*) or a List<CharSequence>, which is similarly wrong /
  meaningless, prior to the base CL http://ag/3345932

VisibilityPreferenceController uses Integer.toString() to
construct corresponding String values, so the test should
be checking for that String value. For consistency with
  testUpdateState_noGlobalRestriction()
this CL uses the equivalent (and slightly shorter)
String.valueOf(int) instead of Integer.toString().

Test: EXPERIMENTAL_USE_OPENJDK9=1.8 make RunSettingsRoboTests \
   ROBOTEST_FILTER=VisibilityPreferenceControllerTest

Change-Id: I48644729d3e0f29d7ffcc981aeef650f2b1426ef