Refactor inputfilter tests

We are currently storing several variables from notify*args inside the
fake policy, in order to check that we received the right values later.
This is inconvenient because it's not clear that these variables are
specifically for the inputfilter test.
Store these in the input event instead.

Bug: none
Test: atest inputflinger_tests
Change-Id: I34dba73eeacf7dd58b10691c6fad3d5206daa780