Make FullBackup::BackupScheme aware of operation type

We currently maintain an instance of FullBackup::BackupScheme
(encapsulates full backup policies, including include / exclude rules)
per package. In line with the new include / exclude config format, where
the rules for each operation types are separate, create BackupScheme per
(package; operation type) pair to  make sure we're only dealing with the
policies relevant to the current operation.

Bug: 174216309
Test: atest FullBackupRulesHostSideTest
Change-Id: I627584186a1d1faf3d699e734e45f5e943ec1f84