InputDispatcher: Fix pointer count when canceling a subset of pointers

There was a bug in the logic for calculating the pointer coords and
properties for canceling a subset of pointers. It was introduced when we
refactored storing the coords in a vector rather than using a
fixed-length array with a pointerCount variable in the following change:
I91b4a88ec5df3f017ade8a6e55be3d3c1281de75

Bug: 346342507
Test: atest inputflinger_tests
Flag: NONE bug fix

Change-Id: Ie9a5fbd5ba1fd75c53b7289a93573ec2d0d1947b