Introduce Exceptionless.Linking.PreserveAttribute to constructors removed by Xamarin Linker (#189)

* Introduce an internal Android.PreserveAttribute and apply it to appropriate constructors. Fixes #188

* Apply PreserveAttribute to assembly instead of constructors.

Author: GeirGrusom

src/Exceptionless/Linking/PreserveAttribute.cs

@@ -0,0 +1,17 @@
+using System;
+
+namespace Exceptionless.Linking {
+    /// <summary>
+    /// This class is used to tell the Xamarin linker to not remove the specified member, type or assembly.
+    /// </summary>
+    internal sealed class PreserveAttribute : Attribute {
+        /// <summary>
+        /// Ensures that all members of this type are preserved.
+        /// </summary>
+        public bool AllMembers;
+        /// <summary>
+        /// Flags the method as a method to preserve during linking if the container class is pulled in.
+        /// </summary>
+        public bool Conditional;
+    }
+}

src/Exceptionless/Properties/AssemblyInfo.cs

@@ -7,7 +7,7 @@
 [assembly: ComVisible(false)]
 [assembly: AssemblyTitle("Exceptionless")]
 [assembly: AssemblyDescription("Exceptionless")]
-
+[assembly: Exceptionless.Linking.Preserve]
 #if !PORTABLE && !NETSTANDARD1_2
 [assembly: Guid("2d458cc4-3bb3-4852-b6a2-11d5ac8672df")]
 #endif