Add scheduled job high water mark metric.

Add a metric to track the maximum number of jobs that are scheduled at a
time, in 30 minute increments.

Bug: 138239687
Bug: 279935506
Test: atest CtsJobSchedulerTestCases:ConnectivityConstraintTest
Test: atest CtsJobSchedulerTestCases:JobSchedulingTest
Test: atest CtsJobSchedulerTestCases:NotificationTest
Change-Id: If0c190bf8ebcca55ef1907020b37a0e1c72181ae