Add ability to start voice interaction session directly

Add internal API's for SystemUI to start a voice interaction session
directly, without using an intent.

Make the assist gesture use that ability, if available.

Change-Id: I88ce3c7514714eb45666884847193585a07417a9
diff --git a/packages/SystemUI/AndroidManifest.xml b/packages/SystemUI/AndroidManifest.xml
index 7a58c87..6a3e89e 100644
--- a/packages/SystemUI/AndroidManifest.xml
+++ b/packages/SystemUI/AndroidManifest.xml
@@ -120,6 +120,9 @@
     <!-- Screen Capturing -->
     <uses-permission android:name="android.permission.MANAGE_MEDIA_PROJECTION" />
 
+    <!-- Assist -->
+    <uses-permission android:name="android.permission.ACCESS_VOICE_INTERACTION_SERVICE" />
+
     <application
         android:name=".SystemUIApplication"
         android:persistent="true"