Return a boolean from the mandatory backup transport setter.

Bug: 73813043
Bug: 64012357
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

Change-Id: I7f47a41070b767a6be8b986be485b4ad782690cf