Refactor include/exclude rules parsing

This pure refactor extracts parsing individual include/exclude tag
parsing into a separate function. A follow up change will introduce
parsing for a new tag. With this refactor that follow up change will be
easier to review.

Bug: 403956528
Test: atest FullBackupTest.java
Flag: EXEMPT refactor
Change-Id: I74f6fb06eafca9ccc24074689a3e25d8830ed067