Hidden SipTransport API definition

The SipTransport API definition, which is being put in using @hide
in order to allow applications to review and build testing around
the definitions.

In Follow up CLs, the APIs will be backed by the appropriate AIDLs
and will gradually start being moved to @SystemApi as they are
implemented and CTS tests are added

Test: CTS, docs review
Bug: 154763999
Change-Id: I5266eb7c077299c5e7a8c7780db6a30fffdfc556