OMS: dump: silently ignore -a

dumpsys will pass -a to each service. Teach the overlay manager to
silently ignore the argument.

Test: adb shell dumpsys | grep --text -A1 -e 'DUMP OF SERVICE overlay:' # no "Unknown argument: -a"
Change-Id: I00a786a8a3cd5996bfac95e05782b2f204f005c5