Set flex policy based on job priority.

Make it possible to have different fallback deadlines and percent drop
rates for each job priority so that higher priority jobs aren't delayed
as significantly as lower priority jobs.

By default:
* HIGH priority jobs have a 6 hour fallback with % drops of 33, 50, 60, 75
* DEFAULT jobs have a 12 hour fallback with % drops of 50, 60, 70, 80
* LOW jobs have a 24 hour fallback with % drops of 50, 60, 70, 80
* MIN jobs have a 48 hour fallback with % drops of 55, 65, 75, 85

The fallback deadline is adjusted based on job execution over the prior
24 hours, with higher priority jobs carrying more weight, in order to
discourage setting a lot of jobs as high priority.

Bug: 236261941
Bug: 299329948
Bug: 299346198
Test: atest CtsJobSchedulerTestCases:FlexibilityConstraintTest
Test: atest frameworks/base/services/tests/mockingservicestests/src/com/android/server/job
Test: atest frameworks/base/services/tests/servicestests/src/com/android/server/job
Change-Id: Ic153e642d7fd45f69986873162f84f69a289313e