Introduce write lock for mDragState

We are going to introduce the drag drop callback however we would not
like to keep holding the window manager lock while invoking the
callback.
The CL introduce separated write lock for mDragState which we can hold
while invoking the upcoming drag and drop callback.

Bug: 65564090
Test: android.server.wm.CrossAppDragAndDropTests, manually check the
      drag and drop behavior on test app.
Change-Id: Icb6484784097bc93c5aa1154c71324aa5353620c