hotspot2: add support for complete PerProviderSubscription/Policy subtree

Added Policy to PasspointConfiguration and the corresponding parser
support in PPSMOParser.

While there, fix a typo in node name "CertSHA256Fingerprint" under
CertificateCredential.

Bug: 34198926
Test: frameworks/base/wifi/test/runtests.sh
Change-Id: Iabe27cd83b6658ed7d4f895d7fe2255fe2094ebb