Early crash of servicemanager shouldn't start apexd

If servicemanager crashes when apexd isn't started yet, apexd shouldn't
be started. Otherwise, apexd will start much earlier than it is supposed
to be. It can cause various issues because apexd has many assumptions
around it (like some directories must have been created, etc.).

Adding `--only-if-running` ensures that.

Bug: N/A
Test: inject an error in servicemanager and see if apexd is not started
upon the servicemanager crash

Change-Id: I0b20a31115aaf7607cc0340e02183871e60d4178