init: Add --only-if-running argument to restart command.

If surfaceflinger crashes before zygote starts, it will restart zygote
too early. Add this argument will allow it to only restart zygote if
it's already running.

Bug: 197224273
Test: manual test
Change-Id: I0ac8dbeaf0a002f8cfcfcc0f63d4b90d08393c5c