Calc job standby runnability based on last job execution...

...not unilaterally on the current time of day.  In practice, the point
is that we should let an app run new jobs immediately if it's been a long
time since it ran any, even if it's in a less-active standby bucket,
because it's being a good citizen.

Jobs for apps in the NEVER bucket (e.g. sync jobs inflated at boot time)
are also treated as immediately runnable.  They won't be run at all until
the app is brought out of standby, so we just treat such apps as good
citizens.

Bug: 63527785
Test: atest CtsJobSchedulerTestCases
Change-Id: If5e3dc18757caf96ba80835f098b48614dda1d9d