SensorInputMapperTest: migrate to InputMapperUnitTest

And make a few other small refactors while I'm at it:

* Use the actual axis constants for setting sensor types and indexes
* Inline methods that are only called once, to keep test context in one
  place
* Move test constants to the anonymous namespace rather than declaring
  and initializing them separately

Test: TEST=inputflinger_tests; m $TEST && \
      $ANDROID_HOST_OUT/nativetest64/$TEST/$TEST \
              --gtest_filter='*SensorInputMapperTest*'
Bug: 283812079
Flag: TEST_ONLY
Change-Id: I4e909c1f4e82e6a83b8c96a1fadeff56f9055861