Use fling tap down gesture as a signal for touchpad contact

... to show the mouse cursor icon immediately upon contact with the
touchpad.

While the gestures library contains kGestureTypeContactInitiated, it is
no longer used. Instead, when there is contact with the touchpad, the
library produces a fling tap down gesture.

The fling tap down gesture can be used as a signal to mark the end of a
previouly started fling event. However, since the fling tap down gesture
being generated roughly corresponds to a stable contact being detected in
the current version of the library, we can use it as an
alias for "contact initiated" as a workaround. This works because the
fling tap down gesture is produced even if there was no fling start
gesture produced in the past.

Whenever we detect that a contact is initiated, we treat is as a move
event of zero magnitude so that the mouse cursor is shown and a hover
event is produced for apps to update their UI.

An alternative approach that was considered was to track touching
pointers from the evdev state, and use that ensure PointerController is
showing the cursor whenever there is a valid pointer. This was
disregarded as it could lead to inconsistencies with the gesture
library. For example, if the gestures library immediately detects a
contact as a palm, we don't want the cursor to be shown.

Bug: 281671810
Test: atest inputflinger_tests
Test: manual, with touchpad
Change-Id: I01dbdd8a139cc6766fcd7e545a961e32237c2af9