Add BackupManager#isBackupServiceActive() system API

So that GMSCore can find out and disable restore flow
and Backup UI.

Bug: 33339643
Test: gts-tradefed run gts -m GtsGmscoreHostTestCases -t com.google.android.gts.devicepolicy.DeviceOwnerTest#testBackupServiceActive (new GTS test in companion CL)
Test: cts-tradefed run cts -m CtsDevicePolicyManagerTestCases -t com.android.cts.devicepolicy.DeviceOwnerTest#testBackupServiceEnabling
Change-Id: I62c38dec2395d3aa926566d128b0067ce7d62c26