Add ANR behavior to javadoc.

Note how JobScheduler behaves when apps don't respond in a timely manner
to onStartJob() and onStopJob().

Bug: 19536175
Test: `m offline-sdk-docs` and view page at out/target/common/docs/offline-sdk/reference/android/app/job/JobService.html
Change-Id: Ib5586801addd3ff83ce19476f98e9ef1cee4e51d