Add method to disable "disable" button in installed app UI

OEMs can now force grey out the "disable" button for important apps on
device.

Change-Id: Ic075a07ad12592bd60238c7b1c9ab84932c8db3c
Fix: 38250742
Test: make RunSettingsRoboTests

src/com/android/settings/applications/ApplicationFeatureProviderImpl.java

@@ -129,6 +129,11 @@ public class ApplicationFeatureProviderImpl implements ApplicationFeatureProvide
         return preferredActivities;
     }
 
+    @Override
+    public Set<String> getKeepEnabledPackages() {
+        return new ArraySet<>();
+    }
+
     private static class CurrentUserAndManagedProfilePolicyInstalledAppCounter
             extends InstalledAppCounter {
         private NumberOfAppsCallback mCallback;