Use JS thread for idleness tracking.

Make sure to use the dedicated JobScheduler thread for all of the
idleness callbacks and state changes.

Bug: 172579710
Test: atest CtsJobSchedulerTestCases:IdleConstraintTest
Test: atest CtsJobSchedulerTestCases:JobThrottlingTest
Change-Id: I269d5927ba2c7b795f61ff73c46c8cd6f56c937c
2 files changed