inputflinger_tests: Put `FakeInputReaderPolicy` in its own file

I would like to be able to put automated tests for the new
`TouchpadEventMapper` in their own file, rather than
InputReader_tests.cpp. To do this I'll need some of the test utilities
in their own files, too.

Bug: 251196347
Test: atest inputflinger_tests
Change-Id: Ibbe27ced5adec2186decd8fb1c359b8ed4cbf870