trim curly braces from UUIDs used as app IDs

[Over in dev-b2g](https://groups.google.com/d/topic/mozilla.dev.b2g/j-r8ed6yAS4/discussion), @ArnaudD reported that an app he was testing in the Simulator had an app:// URL with a GUID containing curly braces, which violates RFC 3986.

@fabricedesre is fixing B2G's app ID generator to trim the braces in [bug 845344](https://bugzilla.mozilla.org/show_bug.cgi?id=845344). We should similarly fix the Simulator's app ID generator.