TouchpadInputMapper: log when producing usage atoms

This should help us debug the flaky CTS tests for this atom.

Bug: 329725372
Test: check the log line appears when running the test:
      $ atest 'android.input.cts.hostside.InputAtomsTest#testTouchpadUsageAtom_FingerAndPalmCounts'
Change-Id: I23346b4904f83abb6cc5a07b687bb0a9802c6bfa