Add meta-data constant for activity->slice

Test: its a constant
Change-Id: I331358bd9619beb413dfa98525b0f2589eaea8ae

core/java/android/app/slice/Slice.java

@@ -57,6 +57,16 @@ public final class Slice implements Parcelable {
             HINT_NO_TINT, HINT_PARTIAL})
     public @interface SliceHint{ }
 
+    /**
+     * The meta-data key that allows an activity to easily be linked directly to a slice.
+     * <p>
+     * An activity can be statically linked to a slice uri by including a meta-data item
+     * for this key that contains a valid slice uri for the same application declaring
+     * the activity.
+     * @hide
+     */
+    public static final String SLICE_METADATA_KEY = "android.metadata.SLICE_URI";
+
     /**
      * Hint that this content is a title of other content in the slice. This can also indicate that
      * the content should be used in the shortcut representation of the slice (icon, label, action),