Make EventEntry const throughout the Dispatcher pipieline

This will help ensure that parts of the event do not change throughout
the dispatching pipieline after they are traced, and that the parts that
do change during dispatch are explicitly tracked.

Bug: 210460522
Test: atest inputflinger_tests
Change-Id: I2738d1a37da7ce72525d59c65f910a6fec09f862
4 files changed