Fixes crash on network status change on Android.

Xamarin.Android has a bug in the network reachability
class that results in a bad crash when the network
status changes.

This works around that bug, which will soon be fixed,
by implementing platform-specific on Android.
Merge pull request #278 from couchbaselabs/restart-async-task-issue

Fixes #277 - Crash on network status change on Android.