[RefactoredBMS] Don't mix "current dataset" tokens for different backup transports

This CL replicates ag/2191170 in RefactoredBMS.

Test: runtest -p com.android.server.backup frameworks-services
Bug: 37973765
Change-Id: Ic3fd528071cef8143e2c3f6b659722f951f0deb9
1 file changed