Fix modifier-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 "modifier"
Change-Id: Idf3c8d13c63770397e895bc05ba2374c10fb3bd1