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.

Bug: 63527785
Test: atest CtsJobSchedulerTestCases
Change-Id: I1521c82f23341246484efa733c43f983a5e9e568