Clarify that getLocalState is always valid for DragEvent

The method getLocalState() always returns valid data
(if the data was stored in startDragAndDrop to begin with),
but the documentation currently states that the events with
action ACTION_DRAG_ENDED do not have valid local state.

Bug: 62067188
Test: bit CtsViewTestCases:android.view.cts.DragDropTest
Change-Id: Ia2f8e5e0bb068aa7ac4aac3f18a5cf01d1a73c32