hotspot2: ignore uninterested headers and content type attributes

Each vendor might put additional headers and content type attributes
in the installation file, and will not be useful for us.  So just
ignore them when parsing the installation file instead of error out.

While there, add support for parsing "Extension" node in PpsMoParser.
This an interior node containing vendor specific information, only
meaningful to the vendor.  So we will ignore fields under this node.

Bug: 36477333
Test: frameworks/base/wifi/tests/runtests.sh
Test: manual test by installing ruckus Passpoint profile
Change-Id: I9954ed000387bc8048d0452a5bfb0f339e676659