Track existing pointers on the virtual touchscreen.

The way we write touch input events use to be stateless, when an action
UP events on a pointer is received, we convert it to a BTN_TOUCH UP
event with that pointer id. However, in multi-touch protocol, an BTN up
is only sent when there's no pointers in touch whereas the lift of a
single pointer is marked by setting tracking id to -1.

This change added a map from the touchscreen to its existing touches, so
that we only send the BTN up when the corresponding entry becomes empty.

Test: atest VirtualTouchscreenTest
Bug: 256172250
Change-Id: I7df865288c0e180db8c6f81c4ad2505913d56492