Add #getTransportFlags to BackupDataOutput

This allows a BackupAgent to check whether the transport has client-side
encryption enabled. It can then use this information to decide whether
to back up more sensitive data.

Bug: 72299360
Test: Manually verified full & kv backup agents receive transport flags
Test: m -j RunFrameworksServicesRoboTests
Change-Id: Ibd9b5f9479815e1721e9d6b7663d892b9ab3fcae