inputflinger_tests: Put `FakePointerController` 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.

Aside from extracting the `FakePointerController` definition into its
own file and putting method bodies into a .cpp file, I've also moved
private members into one place in the header file, and initialized them
there to allow the default constructor to be used.

Bug: 251196347
Test: atest inputflinger_tests
Change-Id: I8f9704c543a07eae5b7135990139ac37eda82ae5