Add test for heuristic palm rejection

For short strokes, the palm rejection code has a heuristic - large
touches are canceled.
Since the behaviour is currently present, let's add an integration test
for it.

Separately, we will also remove 'getLinuxToolType' here because
tool_type is not used in the palm rejection code.

However, tool_code is used in that code. So let's add getLinuxToolCode
instead. This piece does not affect the behaviour of the palm rejection
model, but it makes our inputs more correct.

Bug: 241935838
Test: atest inputflinger_tests
Change-Id: Ied9e185bdfc6e892cf3a1069b98101ca8ae9c74b