Extract KeyboardInputMapperTest and subtests to separate file

The code added to KeyboardInputMapper_test.cpp is identical to that
removed from InputReader_test.cpp, except for the added TODO and copies
of the relevant constants.

Bug: 283812079
Test: atest frameworks/native/services/inputflinger/tests/KeyboardInputMapper_test.cpp
Flag: TEST_ONLY
Change-Id: I33380a3ce5e10db624b061674740f51fb5c2167d

Author: HarryCutts