[New Feature] Show date time picker with a dialog style (configurable)

This change allows you to specify, in `build.yml`, that you want the date/time picker to show up as a dialog on Android. This setting is called `date_time_picker_dialog` under `android`.

The default date/time picker shows up full screen, which is not very pretty on devices with large screens. Setting the theme to dialog makes it dim the WebView and show the date picker as a small box on the screen, which looks a lot better.

Since this is aesthetic, I made this change configurable, and it defaults to the current behavior.