[flutter_local_notifications] Fixes #1486 :Adds ability to count down chronometer with chronometerCountDown

[ebelevics]

Fixes #1486, #791, #306

chronometerCountDown will only work for Android devices that are 7.0 or newer. But this will do the trick for updating timer without updating notification every single second.

AndroidNotificationDetails(
          _channelKey,
          'Channel',
          channelDescription: 'Description',
          when: endAt.millisecondsSinceEpoch, // should be greater than DateTime.now()
          timeoutAfter: endAt.difference(DateTime.now()).inMilliseconds, // this will ensure that notification will hide after reaching 0:00
          usesChronometer: true,
          chronometerCountDown: true, // chronometer will go backwards from positive to negative
        ),

[ebelevics]

Noticed typo at notification_details.dart. Should be /// This property is only applicable to Android 7.0 and newer versions.

[MaikuB]

Thanks for the PR. Could you push an update to fix the issue in the API docs that you had mentioned in your previous message?

[ebelevics]

Api doc corrected

[MaikuB]

Thanks for the update. The unit tests are failing due to the updates made in this PR, could you look to address this as well?

[ebelevics]

Sorry for delay, done.