BridgesList Preferences is an overloaded field

BridgesList is an overloaded field, which can cause some confusion. The list can be:

1. a filter like obfs4 or meek OR  
2. can be a custom bridge

For (1), we just pass back all bridges, the filter will occur elsewhere in the library. For (2) we return the bridge list as a raw stream. If length is greater than 5, then we know this is a custom bridge

We should fix this so that we have separate preferences for 1 and 2. We also shouldn't be looking at string length to determine field type.