implement apple healthkit mindfulness data addition

[cancng]

iOS apps were crashing when saving data in the MINDFULNESS data type. Since the MINDFULNESS data type receives HKCategoryValue.notApplicable i.e. 0 instead of the value, a separate method has been added to fix this error.

[Sebo-the-tramp]

#660 this issue solves everything by setting the value to 0

e.g.

success &= await Health().writeHealthData(value: 0, type: HealthDataType.MINDFULNESS, startTime: DateTime(now.year, now.month, now.day, now.hour, now.minute-15), endTime: now);