Prevent race conditions after dropping database

Block all loaders from accessing the database
before indexing is complete.

Bug: 37501479
Test: make RunSettingsRoboTests
Change-Id: I2af98dcf6bceacbeffa70bd40879c97bb0cbc828
Merged-In: I02f8423c0ffc27abbb8ceb61a8c47d2f0796d0bb

src/com/android/settings/search/IndexingCallback.java

@@ -0,0 +1,12 @@
+package com.android.settings.search;
+
+/**
+ * Callback for Settings search indexing.
+ */
+public interface IndexingCallback {
+
+    /**
+     * Called when Indexing is finished.
+     */
+    void onIndexingFinished();
+}