Fix crash on configuration changes in DatePickerFragment

[unbiaseduser]

Fixes #1201. When you tab in and out of other apps or rotate your screen, the DatePickerFragment won't cause crashes anymore.
Let me explain how the fix works:

• Fragments must have a public, no arguments constructor. Any data you want to pass to them must be put in a Bundle for Android to be able to persist it across configuration changes (like the situations above).
• Communication between fragments or between an activity and a fragment is performed with FragmentManager.setFragmentResultListener() on the activity/fragment that receives the callback and FragmentManager.setFragmentResult() on the one that sends the callback. The callback data must fit in a Bundle.

In the original code, the date picker directly sets the target text field. Here, the code for setting the text field is put in a callback received by the host activity, and the date picker fragment's only responsibility is receiving input data (the Date objects) and sending output data (the picked Date). To know which text field to set, I pass in an "identifier" (which conveniently is the LoyaltyCardField enum that's already used in the "display date picker" method, so I don't have to create a new enum or something.)

[TheLastProject]

This looks great and works well. Thanks for the fix and the detailed explanation! I just have a few nitpicks, nothing big :)

[TheLastProject]

Thanks!

[obfusk]

Thanks for the fix and the explanation. Learned something new :)