Explicitly export content providers

In a future version of Android, the default value for
<provider android:exported> will change from true to false.
Explicitly set android:exported="true"

Bug: 3306452
Change-Id: I8fcce3af6949eb245ea97b6e64c191f465f0ae5f

AndroidManifest.xml

@@ -139,6 +139,7 @@
         <provider
             android:name="com.android.launcher2.LauncherProvider"
             android:authorities="com.android.launcher2.settings"
+            android:exported="true"
             android:writePermission="com.android.launcher.permission.WRITE_SETTINGS"
             android:readPermission="com.android.launcher.permission.READ_SETTINGS" />