Extend XML parser to allow optional requiredFlags attribute

for <include /> rules in <full-backup-content> specification

Give an app developer the option to include files based on the
transport flags exposed by the transport. This allows conditionally
including files as long as the transport identifies itself as
for instance encrypted or device-to-device.

Extend the parsing mechanism to read optional requiredFlags
attributes, and extend existing structures to encompass
that data for BackupAgent to retrieve and act on it
based on FullBackupDataOutput#getTransportFlags().

Bug: 72484288
Test: runtest frameworks-core -c android.app.backup.FullBackupTest
Test: make cts -j40 && cts-tradefed run cts -m CtsBackupHostTestCases -t android.cts.backup.FullbackupRulesHostSideTest
Change-Id: Ibf12995796963f87ed19d9e357ae8546037e5f53