Add setting for assistant

- Add assistant setting in Settings.Secure;
- If the assistant setting is missing, start assistant as current behavior;
- If the setting is present, start assistant use the setting value;

Bug:20210110
Change-Id: I946fbb504452e445879ad688130a1570b857b9c5
diff --git a/core/java/android/provider/Settings.java b/core/java/android/provider/Settings.java
index cac4a53..640f434 100644
--- a/core/java/android/provider/Settings.java
+++ b/core/java/android/provider/Settings.java
@@ -5549,6 +5549,15 @@
         public static final String DOUBLE_TAP_TO_WAKE = "double_tap_to_wake";
 
         /**
+         * The current assistant component. It could be a voice interaction service,
+         * or an activity that handles ACTION_ASSIST, or empty which means using the default
+         * handling.
+         *
+         * @hide
+         */
+        public static final String ASSISTANT = "assistant";
+
+        /**
          * This are the settings to be backed up.
          *
          * NOTE: Settings are backed up and restored in the order they appear