Add search loader for installed apps.

- The loader filters out system apps.
- Loader performs case-insensitive match with app names.
- SearchResultAdapter combines results from multiple loaders into a
  single list.

Fixes: 33347966
Test: make RunSettingsRoboTests
Change-Id: I228ca6fb82f0ac5151b2346c079c2de41104a4df

src/com/android/settings/search2/SearchFeatureProvider.java

@@ -41,4 +41,9 @@ public interface SearchFeatureProvider {
      * Returns a new loader to search in index database.
      */
     DatabaseResultLoader getDatabaseSearchLoader(Context context, String query);
+
+    /**
+     * Returns a new loader to search installed apps.
+     */
+    InstalledAppResultLoader getInstalledAppSearchLoader(Context context, String query);
 }