Java time picker: use ISO date string

I have a Polish user who has an android phone with polish localisation. This caused a bug in the app and I tracked it down to the value of `date.toLocaleString()` which is sent to the javascript callback resulting in an Invalid date in the javascript code. The changes I have made should avoid any localisation problems by using the ISO standard that javascript recognises.

This change is for the TimePicker, so the actual date values being sent to the javascript callback are irrelevant.

[Javascript ISO standard](http://www.ecma-international.org/ecma-262/5.1/#sec-15.9.1.15)