GestureConverter: Reset fake finger on scroll start

The following recent cleanup CL introduced a bug where the fake finger
position was not being reset after a scroll gesture ends:
I02bd7c8f4c4126c9ae7d2fdffd94175b37478925

Fix the newly introduced bug and add a test case.

Fixes: 340417860
Test: atest inputflinger_tests
Change-Id: I747ba4ace6d09424c10925cdd7a1f4b0ad6a6249