Fix Javadoc-related lint erros under a.i.navigationbar

This is a mechanical cleanup CL.  There should be no behavior change.

Bug: 215443343
Test: prebuilts/checkstyle/checkstyle.py -f \
  frameworks/base/core/java/android/inputmethodservice/navigationbar/ \
  | grep "Missing a Javadoc comment"
Change-Id: I5cb365b69864a145e7eaf7ff4454c261324ab845