Differentiate fused and unfused external styluses

An external stylus can be "fused" with touch data if it reports
pointer-specific data independently from the touch device. The only such
data we currently support is pressure. Thus, a fused external stylus
will reports pressure data.

If a fused stylus is connected, we withold all touches for up to 72
milliseconds to wait for pressure data from the stylus. If we get
pressure data within this time, we assume that the first pointer that
went down is actually the stylus, and fuse the pressure information from
the stylus to the pointer.

If an external stylus does not report pressure, it is an "unfused"
stylus.

When such an external stylus is connected, we don't withold any touches.
We still report button presses from these styluses through the touch
device.

DD: go/android-stylus-buttons

Bug: 246394583
Test: atest inputflinger_tests
Change-Id: I3d687a10630019756170e7e5e5f5d1902eb96e36