Remove SyncManager's local job cache

SyncManager maintains a local copy of all
scheduled syncs. This was done so that we
don't have to query JobScheduler every time
we need to go through the syncs to reschedule
them, etc. Keeping JobScheduler's job list and
the SyncManager's copy in sync is messy. Not
keeping a copy with SyncManager would also
allow JobScheduler to drop jobs based on an
app being uninstalled or other external events.

Here, a function to query all pending jobs
scheduled by the system process is exposed
from JobScheduler and SyncManager uses it
instead of maintaining a copy of its own.

Change-Id: I723dbb3835a0f9c7e8844483004e7b0f7f340daf