mobile-config-prefs.js: Move pipewire camera setting from default prefs

Using the default preferences for some reason results in the setting showing up as enabled in about:config - but not actually enabling the Pipewire webrtc backend. Using pref() instead works - thus let's do that for now.

Closes webrtc-preview dialog almost always does not sh... (#134 - closed)

See also mobile-config-firefox breaks camera selection o... (pmaports#4695 - closed)