Improve idmap2 xml parsing

To parse the <overlay> tags for internal overlay references, we require
the ability to parse XML. The current implementation only allows idmap2
to find a tag somewhere in an xml file.

This implementation allows for the retrieval of an iterator that
iterates over direct children xml elements. Now we can enforce that the
<overlay> tag in the manifest is nested within the <manifest> tag and we
can ensure <item> tags are within the <overlay> tags of the overlay
resource configuration xml.

Bug: 135051420
Bug: 135943783
Test: idmap2_tests
Change-Id: I12f965b55c18960af6cf9dc6ca46429f53124c9a