Fix file matching w/ full-backup rules xml

Documentation is pretty vague:
https://developer.android.com/guide/topics/data/autobackup#XMLSyntax.

But there were a couple of issues:
* It was prematurely returning false without consuming the rest of the
  includes (cause of the bug linked).
* It was using string comparison for checking if a file is in a
  directory, which ended up flagging directories such as "a/b" as
  containing files "a/b.txt".

Reviewers,

* Please, pay full attention to test cases.
* Since this is code move + code change, set diff as 2..latest to check
changes to the function.

Bug: 110720194
Test: atest BackupUtilsTest
Test: Backup and restore app w/ multiple directory includes, verify
      everything restored

Change-Id: Ic0fea43156ce8fb641af69ae73679289a20c291c