sink: Add portable form factor priority (450)

sink: Add portable form factor priority (450)

The 'portable' form factor was currently missing meaning it is not
getting any form-factor priority at all and it would therefore always
be ranked lower then internal devices (which receive 400 form factor
priority). The priority 450 is smaller then 'speaker', based on the
idea that a portable device might have less quality then a dedicated
'speaker' device (some Yamaha amplifiers announce themselves as such).

https://bugs.freedesktop.org/show_bug.cgi?id=100579