New management API for making backups mandatory.

Let the device owner make backups with a chosen backup
transport mandatory.

BUG: 64012357
Test: make RunFrameworksServicesRoboTests
Test: manually together with the corresponding GmsCore change.
Test: cts-tradefed run cts -m CtsDevicePolicyManagerTestCases --test
com.android.cts.devicepolicy.DeviceOwnerTest#testGetAndSetMandatoryBackupTransport
Test: cts-tradefed run cts -m CtsBackupHostTestCase --test
android.cts.backup.BackupDeviceOwnerHostSideTest#testMandatoryBackupTransport

Change-Id: I9bfae5799beae3459659e697813b75a9b508ae55