Add an API to query pending job reason.

With Data Transfer jobs, an app may want to tell the user why the
transfer isn't running. This adds a simple API that lets the app
query why a job may isn't running. It will only return one reason even
if there are multiple reasons the job isn't running.

Bug: 255371817
Test: atest CtsJobSchedulerTestCases:JobSchedulingTest
Change-Id: Ie86079c63a94312007406a43c4883e18d39a00ab