Cupertino TextField Cursor Fix (#27697)

* Fix for #26261. Changes CupertinoTextField's cursorColor to read from CupertinoTheme instead of prior default of activeBlue. CursorColor will still default to activeBlue for light theme and activeOrange for dark theme if a primary color has not been specified for the CupertinoTheme.

* Reverted unnecessary changes in XCode file.

* Updated text_field.dart per suggestions from @gspencergoog

* Updated comments for cursorColor to reflect appropriate hyperlinks per @Hixie

* Simplified cursorColor assignment per @xster

* Added test in cupertino/text_field_test.dart to check for correct cursorColor based on CupertinoTheme per @Hixie & @xster.