Add TestApis for updateTransportAttributes GTS tests

These TestApis are getters to access BackupManagerService transport
attributes. This is to validate that BackupManager.updateTransportAttributes
system api succeeds when testing in GTS (see ag/3615301).

Bug: 72485407
Test: gts-tradefed run gts -m GtsBackupHostTestCases -t com.google.android.gts.backup.BackupManagerTransportAttributesHostSideTest
Change-Id: I0edb1aa0fd776e062f800cf7a79de5cd2e2436df