Count active touches in MultiTouchMotionAccumulator

Counting active touches will allow us to detect first touch on the
device. This information will be used by mapper to getect start of a new
gesture, when the first touch is detected.

Bug: 301055381
Test: atest MultiTouchMotionAccumulatorTest

Change-Id: I53f65daef3e5aa324f42eec6aa85ed6d5ef01bf5
4 files changed