Write support on Google Fit and HealthKit

[matsunanaro]

This PR is derived from the PR #182 by Matthew Pitzer.
On top of the original PR, this PR

• implemented the writeHealthData method on Android using Google Fit API.

  ///
  /// Saves health data into the HealthKit or Google Fit store
  ///
  /// Returns a Future of true if successful, a Future of false otherwise
  /// 
  /// Parameters
  /// 
  /// [value]  
  ///   value of the health data in double
  /// [type]   
  ///   the value's HealthDataType 
  /// [startTime] 
  ///   a DateTime object that specifies the start time when this data value is measured. 
  ///   It must be equal to or earlier than [endTime]
  /// [endTime]
  ///   a DateTime object that specifies the end time when this value is measured.
  ///   It must be equal to or later than [startTime].
  ///   Simply set [endTime] equal to [startTime] 
  ///   if the value is measured only at a specific point in time.
  /// 
  Future<bool> writeHealthData(double value, HealthDataType type,
      DateTime startTime, DateTime endTime)

• fixed issue [Health version: 3.0.5] crash in SwiftHealthPlugin.getData #393 by returning the result in UI main thread. Please refer to [Health version: 3.0.5] crash in SwiftHealthPlugin.getData #393 for more details
• fixed issue health 3.1.1+1 : the getHealthDataFromTypes method returns mmoll for BLOOD_GLUCOSE on Android #423 of by adding unit conversion from and to mgdl, as mmoll is the storage unit used in Google Fit.
• reinstated the Metric units as mentioned in the original PR's comments
• refactored code in HealthPlugin.kt

[bardram]

Thanks for this PR - will merge it and release.