Add integration tests to verify the behavior of a fused external stylus

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: I2c00fca0f5e3d5214ebb2d0af04ed4efe14de9f7
6 files changed