Fix JobSchedulerService FBE regression.

Internally isReadyToBeExecutedLocked() needs to check if the target
service is actually present and runnable.  For example, when an FBE
device is locked, we need to keep any encryption-unaware jobs
pending until the device is later unlocked.

Bug: 26279465
Change-Id: I53ff4a2243ebe8a199d0e8dcf87dc3f5b06a2686