Clarify unbuffered dispatch may fail before attached to window

Add the docs to to clarify that prior to Android U, the call to
`View::requestUnbufferedDispatch` will fail if the View is not
attached to window.

Bug: 268105812
Test: manual
Change-Id: Ic57ac42447750129a80f6aa71710963ae4594ce0

core/java/android/view/View.java

@@ -17167,6 +17167,9 @@ public class View implements Drawable.Callback, KeyEvent.Callback,
      * This is similar to {@link View#requestUnbufferedDispatch(MotionEvent)}, but does not
      * automatically terminate, and allows the specification of arbitrary input source classes.
      *
+     * <p>Prior to {@link android.os.Build.VERSION_CODES#UPSIDE_DOWN_CAKE}, this method will fail
+     * when this View is not attached to a window.
+     *
      * @param source The combined input source class to request unbuffered dispatch for. All
      *               events coming from these source classes will not be buffered. Set to
      *               {@link InputDevice#SOURCE_CLASS_NONE} in order to return to default behaviour.