On both Android and iOS (when not using a simulator) the application
procees doesn't have write permissions to the default lnd directory.
This commit adds a note about using the app directory given by the used
platform.
The wallet unlocker service also requires the TLS certificates to be
added, but this was not set. This commit sets the options similar to
what is done for the regular RPC server.
Use the custom listeners' Ready channels to provide callbacks to the
mobile application when the gRPC services are ready.
NOTE: this changes the Start() API by adding one extra callback.
