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/ApplicationFeatureProvider.java

@@ -24,6 +24,7 @@ import android.view.View;
 import com.android.settings.applications.instantapps.InstantAppButtonsController;
 
 import java.util.List;
+import java.util.Set;
 
 public interface ApplicationFeatureProvider {
 
@@ -86,6 +87,11 @@ public interface ApplicationFeatureProvider {
      */
     List<UserAppInfo> findPersistentPreferredActivities(@UserIdInt int userId, Intent[] intents);
 
+    /**
+     * Returns a list of package names that should be kept enabled.
+     */
+    Set<String> getKeepEnabledPackages();
+
     /**
      * Callback that receives the number of packages installed on the device.
      */