Fix touchpad palm filtering logic

When I added this logic [0], I was under the mistaken impression that
the Gesture library required touches that were leaving the pad to be
reported as FingerStates with a tracking ID of -1. It turns out that it
doesn't, and in fact such FingerStates shouldn't be sent to it [1]. This
makes the palm filtering logic simpler, since we can just always ignore
a slot if it has the palm tool type, rather than having to make one last
FingerState for it if was previously a finger.

[0]: Change ID I7d898889d4bf4e8068194d22150c02a69b59c435
[1]: https://crrev.com/c/4360158

Bug: 251196347
Bug: 270041770
Test: atest inputflinger_tests:HardwareStateConverterTest
Test: limited manual testing
Change-Id: I7f70f1b677ba19a472fd1670918a82dfe65c6f39