Fix ghost pointer problem when transitioning from dragging to delegating.

We were not sending ACTION_DOWN to close out the dragging event stream before we passed control to talkback to let it determine what to do.

Flag: EXEMPT bugfix
Fix: 300002193
Test: Manual. Scroll with two fingers, then place a third finger on the screen. Inspect the event stream.
Change-Id: I8a6cd033874c7e73aa781e7eda59285d950d202f