Added priority to JobScheduler scheduling

Priority can be assigned to jobs. Higher priority
jobs can preempt lower priority ones. Reason for
calling onStopJob (timeout, preempt, etc.) is set
on the JobParameters object.

Reference:
https://docs.google.com/document/d/1fuVO5rBCkODx8wjk6uulFCP1Uzfx7IVsw2EyKKrGqVA

Change-Id: Ic36016514cec076984d44086316d8d00d896b3aa