remove possible JobScheduler race in cancel()

Client can jobFinished() before getting a cancel msg.
1) Do better clean up of JobServiceContext after client jobFinished()
to remove superfluous MSG_CANCELs
2) When processing MSG_CANCEL check whether the context is still active
3) Do JobServiceContext cleanup before calling back to JobSchedulerService
Client can get a cancel msg even after calling jobFinished() (opposite to above)
1) explicitly check whether there are any MSG_CALLBACKs in the queue before
processing a MSG_CANCEL. If there are we can throw away the cancel.

Bug: 16547638
Change-Id: I90644586c7895a9ce97de752a5d657faf7f74b78