inputflinger_tests: Put `InstrumentedInputReader` 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.

As well as moving the code to separate .h and .cpp files, the private
members of `FakeInputReaderContext` were also moved to the bottom of the
definition and the `private:` made explicit.

Bug: 251196347
Test: atest inputflinger_tests
Change-Id: Iad4cb00fc23ecac0e2f71ea21481585fcc80c92f