OMS: fix checkstyle warnings

Fix checkstyle warnings related to the overlay manager, i.e. in the Java
files under

  - core/java/android/content/om
  - core/tests/overlaytests
  - services/core/java/com/android/server/om

Test: prebuilts/checkstyle/checkstyle.py -f $path
Change-Id: I5078629240da2ceb1fd710d2e79e3114bafdd5bb