inputflinger_tests: Put `FakeEventHub` 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 `FakeEventHub` definition into its own file
and putting method bodies into a .cpp file, I've made a few other minor
refactors:

* Use an early return to reduce nesting in `markSupportedKeyCodes`
* Use a ternary instead of an `if` in `getScanCodeState`
* Remove some single-use `device` variables and replace their use with a
  call to `getDevice`
* Reordered the methods in the .h to make the grouping a little more
  logical
* Tried to follow "Include What You Use" in the new files

Bug: 251196347
Test: atest inputflinger_tests

Change-Id: I55fe8267976d7aba58e1e4067b041d92dfd5347d

to fixup

Change-Id: I59df4e03db7e30cf7d17425ed38fe7f639dec836