Fix issue #21621920: VI: need mechanism to get current request

Add new APIs to associate a Request with a name, get all active
requests, and get active request by name.

Also add a new Activity.onProvideReferrer() which will allow
applications to propagate referrer information to the assistant
(and other apps they launch) in a consistent way.

Change-Id: I4ef74b5ed07447da9303a74a1bdf42e4966df363
diff --git a/core/java/android/content/Intent.java b/core/java/android/content/Intent.java
index 5c23204..25be96a 100644
--- a/core/java/android/content/Intent.java
+++ b/core/java/android/content/Intent.java
@@ -1226,7 +1226,8 @@
      * <p>
      * Input: {@link #EXTRA_ASSIST_PACKAGE}, {@link #EXTRA_ASSIST_CONTEXT}, can provide
      * additional optional contextual information about where the user was when they
-     * requested the assist.
+     * requested the assist; {@link #EXTRA_REFERRER} may be set with additional referrer
+     * information.
      * Output: nothing.
      */
     @SdkConstant(SdkConstantType.ACTIVITY_INTENT_ACTION)