Touchpad: correct HardwareState::touch_cnt for palms

The palm filtering logic previously only reduced the
HardwareState::finger_cnt field when the touchpad had identified palms,
not touch_cnt. I suspect that this was leading the Gestures library to
go into T5R2 mode (for touchpads that sometimes report more touches than
they can track), meaning that palms sometimes caused tap-to-clicks. I
can't reproduce this exact issue myself, but correcting this
inconsistency is good to do anyway.

Bug: 275616121
Test: atest inputflinger_tests:HardwareStateConverterTest
Test: check debug logs to verify palms don't cause touch_cnt to increase
Change-Id: Ic7b22864214351875afe9969fdbaab42db9d7877