[msbuild]: Add TLS Provider selection for BTLS.

* Tasks/BuildApk.cs: Add 'TlsProvider' parameter.

* Xamarin.Android.Common.targets: Add <AndroidTlsProvider> property
  and pass it to BuildApk.

Add this to your .csproj file to select the TLS Provider.  Valid
values are "legacy" and "btls".

We now add a new 'XA_TLS_PROVIDER' environment variable to the .apk,
which is read by System/Mono.Net.Security/MonoTlsProviderFactory.Droid.cs
to select the TLS Provider.

Linker support (to eliminate the unused provider, similar to how it's
already done in xamarin-macios) is coming shortly.

src/Xamarin.Android.Build.Tasks/Tasks/BuildApk.cs

@@ -85,6 +85,7 @@ namespace Xamarin.Android.Tasks
 		public string AndroidGdbDebugServer { get; set; }
 		public string AndroidEmbedProfilers { get; set; }
 		public string HttpClientHandlerType { get; set; }
+		public string TlsProvider { get; set; }
 
 		[Output]
 		public ITaskItem[] OutputFiles { get; set; }
@@ -201,6 +202,7 @@ namespace Xamarin.Android.Tasks
 			Log.LogDebugTaskItems ("  LibraryProjectJars:", LibraryProjectJars);
 			Log.LogDebugTaskItems ("  AdditionalNativeLibraryReferences:", AdditionalNativeLibraryReferences);
 			Log.LogDebugTaskItems ("  HttpClientHandlerType:", HttpClientHandlerType);
+			Log.LogDebugTaskItems ("  TlsProvider:", TlsProvider);
 
 			Aot.TryGetSequencePointsMode (AndroidSequencePointsMode, out sequencePointsMode);
 
@@ -323,6 +325,7 @@ namespace Xamarin.Android.Tasks
 			const string defaultLogLevel = "MONO_LOG_LEVEL=info";
 			const string defaultMonoDebug = "MONO_DEBUG=gen-compact-seq-points";
 			const string defaultHttpMessageHandler = "XA_HTTP_CLIENT_HANDLER_TYPE=Xamarin.Android.Net.AndroidClientHandler";
+			const string defaultTlsProvider = "XA_TLS_PROVIDER=default";
 			string xamarinBuildId = string.Format ("XAMARIN_BUILD_ID={0}", buildId);
 
 			if (Environments == null) {
@@ -340,6 +343,7 @@ namespace Xamarin.Android.Tasks
 			bool haveMonoDebug = false;
 			bool havebuildId = false;
 			bool haveHttpMessageHandler = false;
+			bool haveTlsProvider = false;
 
 			foreach (ITaskItem env in Environments) {
 				environment.WriteLine ("## Source File: {0}", env.ItemSpec);
@@ -356,6 +360,8 @@ namespace Xamarin.Android.Tasks
 					}
 					if (lineToWrite.StartsWith ("XA_HTTP_CLIENT_HANDLER_TYPE=", StringComparison.Ordinal))
 						haveHttpMessageHandler = true;
+					if (lineToWrite.StartsWith ("XA_TLS_PROVIDER=", StringComparison.Ordinal))
+						haveTlsProvider = true;
 					environment.WriteLine (lineToWrite);
 				}
 			}
@@ -373,6 +379,8 @@ namespace Xamarin.Android.Tasks
 
 			if (!haveHttpMessageHandler)
 				environment.WriteLine (HttpClientHandlerType == null ? defaultHttpMessageHandler : $"XA_HTTP_CLIENT_HANDLER_TYPE={HttpClientHandlerType.Trim ()}");
+			if (!haveTlsProvider)
+				environment.WriteLine (TlsProvider == null ? defaultTlsProvider : $"XA_TLS_PROVIDER={TlsProvider.Trim ()}");
 			
 			apk.AddEntry ("environment", environment.ToString (),
 					new UTF8Encoding (encoderShouldEmitUTF8Identifier:false));

src/Xamarin.Android.Build.Tasks/Xamarin.Android.Common.targets

@@ -2243,7 +2243,8 @@ because xbuild doesn't support framework reference assemblies.
     LibraryProjectJars="@(ExtractedJarImports)"
     AndroidEmbedProfilers="$(AndroidEmbedProfilers)"
     AndroidGdbDebugServer="$(AndroidGdbDebugServer)"
-    HttpClientHandlerType="$(AndroidHttpClientHandlerType)">
+    HttpClientHandlerType="$(AndroidHttpClientHandlerType)"
+    TlsProvider="$(AndroidTlsProvider)">
     <Output TaskParameter="BuildId" PropertyName="_XamarinBuildId" />
     <Output TaskParameter="OutputFiles" ItemName="ApkFiles" />
   </BuildApk>