Otapreopt: Allow A/B dexopt only when bootcomplete

Under certain circumstances cmd may not return because it is waiting
for a service manager. Avoid this situation by restricting the otapreopt
script to only do work when the system has completed boot (which
means that a service manager must exist).

Bug: 30797145
Change-Id: Ic1445e20f1ca06fb38e7707771bde818836c3adc