Add xml source for network security configuration

XmlConfigSource parses an ApplicationConfig from an xml resource.
Currently this supports app-wide default configuration via the
base-config element, per domain via the domain-config element and
inheritance of unset properties at parse time.

Inheritance of unset properties is currently only:
domain-config -> base-config -> platform default configuration
Where the most specific value is used.
For example: If the base-config specifies trust anchors, all connections
will use those anchors except for connections to a domain which has a
domain-config that specifies trust anchors, in which case the
domain-config's trust anchors will be used. If the domain-config or
base-config don't set trust anchors, or don't exist, then the platform
default trust anchors will be used.

Nested domain-config entries, debug-overrides, and thorough
documentation of the xml format will follow in later commits.

Change-Id: I1232ff1e8079a81b340bc12e142f0889f6947aa0