Fix AppRestrictionsHelperTest.

Recently mockito has been updated to version 2.7.* and any(*.class) will now
reject null values.

Bug: 36531717
Test: runtest -x packages/SettingsLib/tests/integ/src/com/android/settingslib/users/AppRestrictionsHelperTest.java
Change-Id: Ia3ff2f470a95f6ee36a97eba8699425274500f17