Fix unreliable Preferences from Account Source (#798)

* Create empty Preferences object on error

* Update prefs from account when preferences fails

mastodon/src/main/java/org/joinmastodon/android/api/session/AccountSession.java

@@ -146,6 +146,9 @@ public class AccountSession{
 					@Override
 					public void onError(ErrorResponse error){
 						Log.w(TAG, "Failed to load preferences for account "+getID()+": "+error);
+						if (preferences==null)
+							preferences=new Preferences();
+						preferencesFromAccountSource(self);
 					}
 				})
 				.exec(getID());