Refactor XML config parsing

Do a small refactor of include / exclude rules parsing. In a follow-up
CL we will introduce the logic that parses the new include / exclude
config format. To ensure some of the code can be reused, this CL
extracts parts of parseBackupSchemeFromXmlLocked() function into
separate functions.

Test: atest FullBackupTest
Bug: 174216309
Change-Id: I01e6bcfe4b772f35ece4b8cb21b2e003c28ae55c