Add documentation about pressure to VirtualTouchEvent

The underlying input devices are configured with a pressure range of
0-255. MotionEvent uses a normalized range of 0.0-1.0, which has caused
some confusion. This change adds documentation about the expected range.

Test: Documentation only change
Bug: 259554911
Change-Id: Iddb6cd5b5cd23bb15f75620041d7d3cbf49959db