GestureConverter: add finger count axis for multi-finger swipes

Consumers of multi-finger swipes (i.e. SysUI) previously had to wait
until the first MOVE event to find out how many fingers were swiping,
even though in the framework we knew this when sending the DOWN event.
Adding this as an axis on the DOWN event should let SysUI simplify their
code.

Bug: 283093437
Test: atest inputflinger_tests:GestureConverterTest
Test: modify InputDispatcher to send multi-finger swipes to apps too,
      then check reported values in a test app
Change-Id: I34d1fdf096c49d7eb9b5d8ebd64427eb4e5db1f4