Non incremental backup flag for requestBackup

This cl adds a new requestBackup API to
BackupManager that takes in an int flag
to indicate whether the caller wants the
entire key value set to be passed to the
transport and not just a diff.

Change-Id: Ia225797a58c4431fe742f2f116b257d006b30cd1

Bug: 33749084
Ref: go/request-backup-api-changes
Test: GTS Test at ag/1774002