Add unit tests for external stylus fusion

Add unit tests to demystify the external stylus fusion process and to
document its current behavior.

A fused external stylus reports pressure through a separate input
device. That pressure information is then fused with touches from the
touchscreen.

Bug: 246394583
Test: atest inputflinger_tests
Change-Id: I289e4470eb9383bc9e203bff8f76609b0d533e51