Move refresh prekeys out of SignalProtocolStore

Use an event/listener instead

// FREEBIE

js/background.js

@@ -21,6 +21,9 @@
     // start a background worker for ecc
     textsecure.startWorker('/js/libsignal-protocol-worker.js');
     Whisper.KeyChangeListener.init(textsecure.storage.protocol);
+    textsecure.storage.protocol.on('removePreKey', function() {
+        getAccountManager().refreshPreKeys();
+    });
 
     extension.onLaunched(function() {
         console.log('extension launched');