Fix JobScheduler CTS tests

The JobScheduler CTS tests were making implicit assumptions about alarm
manager semantics; in particular, about the ability to schedule alarms in
rapid succession.  This broke when the platform behavior changed out from
under the tests.

The job scheduler tests now accommodate much more relaxed execution
timing.

Bug 18928665

Change-Id: I09383eebbc64fec9e6fafe73a4df5da8efe95bc0