Adding buffer time for periodic jobs run near the end of their windows.

If a periodic job finishes close to the end of its window, there's a
chance it could run soon afterwards given its next window starts soon.
Adding a buffer window of 1/6 period to avoid having periodic jobs run
too soon after a successful run.

Bug: 123248627
Test: atest com.android.server.job.JobSchedulerServiceTest
Test: atest CtsJobSchedulerTestCases
Change-Id: I8da2847c9383770a49ef404c0e0f7e88e4c2366f