wifi: hotspot2: omadm: add XML parser for parsing OMA-DM XML string

The XML parser will parse an XML string into a tree-like structure
represented by XMLNode, which will be used by an object specific parser
(e.g. PerProviderSubscription Management Object tree parser) for further
processing.

Bug: 32129686
Test: frameworks/base/wifi/tests/runtests.sh

Change-Id: I69c1931b2e8a6f3551e4eba5cffa3d2d0877872a