Refactor input code for require_kernel_config parameter

As part of the next commit to allow kl files to specify a required
kernel config, some small refactorings were done. Move those here to a
separate CL to make it easier to review.

Bug: 228005926
Test: atest libinput_tests
Change-Id: Iab06bb6ef44807308ee2b3e6b8a0c780bb748f09