defaults: Lower priority for src:webkit for non-amber phone

Debian thankfully ships security updates for webkit but they don't use WPE so break epiphany. Hence lower the priority of packages from amber, amber-security, amber-updates and amber-proposed-updates but leave amber-phone, amber-phone-staging at default priority.

This still allows to use

apt install /amber-security

while prefering amber-phone{,-staging} by default.

/cc @alexander.mikhaylenko