Hide mandatory backup's API.

Bug: 77801801
Bug: 73750212
Bug: 64012357

Test: make RunFrameworksServicesRoboTests
Test: cts-tradefed run cts -m CtsDevicePolicyManagerTestCases --test
com.android.cts.devicepolicy.DeviceOwnerTes
cts-tradefed run cts -m CtsBackupHostTestCase

Change-Id: I7699755309b3fbff7c3fec2a06a0f409fd4087de