res_pjsip: Fix codec preference defaults.

When reading in a codec preference configuration option
the value would be set on the respective option before
applying any default adjustments, resulting in the
configuration not being as expected.

This was exposed by the REST API push configuration as
it used the configuration returned by Asterisk to then do
a modification. In the case of codec preferences one of
the options had a transcode value of "unspecified" when the
defaults should have ensured it would be "allow" instead.

This also renames the options in other places that were
missed.

Change-Id: I4ad42e74fdf181be2e17bc75901c62591d403964