Bootstrap installed system apps only when the target user is running

I.e. don't bother trying to send a BOOT_COMPLETED broadcast to the
newly-installed/enabled system app when its host user isn't live.

Also use the static isSystemApp()-type helpers instead of duplicating
those tests locally.

Bug 8594153

Change-Id: I4bd0b86351d67f72c44eae8d1afce6a98f034d31