SmsHeader rewrite, in preparation for migration to public API.

See http://b/issue?id=1751571

Changes the semantics of SmsHeader from containing only opaque data
to exposing occurs-once frequently-used fields together with a list
of opaque fields.  Also changes the coding to and from byte array to
be symmetrical, whereas previous encoding had an extra length field.

Cdma SmsMessage -- cleanup of code paths along with adjustments to
match the new header semantics, which should address at least some
of the issues with concatenated messages.  See http://b/issue?id=1809759