I assume that the native calendar application is not open source, so I doubt that you will find the source somewhere on the Internet.
But you can probably solve this by creating a custom adapter and inflating different layouts for getView() (current / selected item) and getDropDownView() (spinner strings).
Edit: I just saw that the calendar application has become open. Here is git-Link: https://android.googlesource.com/platform/packages/apps/Calendar.git
I would also be curious if the extended ActionBar navigation bar would be a template or custom widgets. Would you publish your results after checking the source?
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