Fix DismissView cannot be clicked sometimes after boot

mActivePointerId should be initialized to INVALID_POINTER.
Otherwise, the click event to DismissView can be intercepted by
onInterceptTouchEventScroll() due to wrong calculated value of yDiff
when mActivePointerId is not intialized.

This issue can be reproduced at the first click to DismissView while
user have not touched any notification views ever after boot.
And that click event must contains some MOVE events.

Test: manual - Check whether user always can click DismissView
               after boot.
Fixes: 31649735
Change-Id: If9baf6db1b891a58e17153806d52d395ddd661df