Fix [kv-4] PerformBackupTask possible NPE/Wrong package report.

mCurrentPackage is null when package doesn't exist, which lead test
failed with NPE
Use request.packageName for case BackupTransport.AGENT_UNKNOWN instead

Bug: 77272500

Test: m -j RunFrameworksServicesRoboTests ROBOTEST_FILTER=PerformBackupTaskTest
Change-Id: I27be85f0c3f95e36edd92d0d552189fbcf6952dd