[Core, Android] Fix to address the elusive 'Sharing violation on path PropertyStore.forms.tmp' (#1075)

* use a semaphor for actual saving piece
make reading serialized propertystore readonly to help with collisions.

* change over to async Task and await OnStateChanged() method to try and mitigate what might be the race condition causing the ' Sharing violation on path PropertyStore.forms.tmp' error

* update Semaphore name

* remove private specifier per the style guide

* Update docs and add warning suppression for broken mono warning

* ... and adding the warning suppression to Release mode, as well.

* -add try...finally block so we can guarantee that the semaphore is released if there's an exception while saving the properties.
-remove the legacy code that the semaphore replaces

* Update docs

* Attempting to fix docs

* Trying again to get docs to pass

1 files changed, 12 insertions, 12 deletions

diff --git a/Xamarin.Forms.Core/Application.cs b/Xamarin.Forms.Core/Application.cs
index 5df8ad6c..b084b52a 100644
--- a/Xamarin.Forms.Core/Application.cs
+++ b/Xamarin.Forms.Core/Application.cs
@@ -1,6 +1,7 @@
 using System;
 using System.Collections.Generic;
 using System.Collections.ObjectModel;
+using System.Threading;
 using System.ComponentModel;
 using System.Threading.Tasks;
 using Xamarin.Forms.Internals;
@@ -15,14 +16,13 @@ namespace Xamarin.Forms
 		readonly Lazy<PlatformConfigurationRegistry<Application>> _platformConfigurationRegistry;
 
 		IAppIndexingProvider _appIndexProvider;
-		bool _isSaving;
 
 		ReadOnlyCollection<Element> _logicalChildren;
 
 		Page _mainPage;
 
 		ResourceDictionary _resources;
-		bool _saveAgain;
+		static SemaphoreSlim SaveSemaphore = new SemaphoreSlim(1, 1);
 
 		protected Application()
 		{
@@ -304,16 +304,16 @@ namespace Xamarin.Forms
 
 		async Task SetPropertiesAsync()
 		{
-			if (_isSaving)
-			{
-				_saveAgain = true;
-				return;
-			}
-			_isSaving = true;
-			await DependencyService.Get<IDeserializer>().SerializePropertiesAsync(Properties);
-			if (_saveAgain)
-				await DependencyService.Get<IDeserializer>().SerializePropertiesAsync(Properties);
-			_isSaving = _saveAgain = false;
+			await SaveSemaphore.WaitAsync();
+            try
+            {
+                await DependencyService.Get<IDeserializer>().SerializePropertiesAsync(Properties);
+            }
+            finally
+            {
+                SaveSemaphore.Release();
+            }
+
 		}
 
 		class NavigationImpl : NavigationProxy