ENH: Replaces transport config with namespaced settings
This MR contains the following:
- Adds a new guice module: SettingsManager, that abstracts the settings interface from the underlying database
- Uses SettingsManager to replace the db.getConfig/mergeConfig calls in the Settings activity
- Replaces the getSettings and mergeSettings calls in the database modules to include a namespace parameter. Updates the corresponding queries in the Jdbc module
- Removes the TransportConfig module, as it is not used anymore
- Updates the DuplexPlugin interface, to reflect this change in schemas and calls
- Updates the calls in the corresponding plugins, for both briar-android and briar-desktop
- Updates the hardcoded plugins in the tests to reflect these changes and updates the unit tests.