[Java.Interop] Make Java.Interop.Export use optional (#401)

The probing for `Java.Interop.Export.dll` performed within
`JniRuntime.SetMarshalMemberBuilder()` can be costly if the
`Java.Interop.Export.dll` assembly isn't present or cannot be found,
which is *always* the case (currently) in Xamarin.Android, adding --
depending on device and profiler in use -- ~100ms to app startup (!).

Add a new `JniRuntime.CreationOptions.UseMarshalMemberBuilder`
property which controls whether `Java.Interop.Export.dll` is loaded
*at all*.  If true -- the default -- then `JniRuntime` *will* attempt
to load `Java.Interop.Export.dll`, allowing the
`JniRuntime.MarshalMemberBuilder` property to not throw a
`NotSupportedException`.

If `JniRuntime.CreationOptions.UseMarshalMemberBuilder` is false --
which will become the default in Xamarin.Android -- then
`JniRuntime.SetMarshalMemberBuilder()` will do nothing.

Author: jonpryor

src/Java.Interop/Java.Interop/JniRuntime.JniMarshalMemberBuilder.cs

@@ -10,6 +10,7 @@ namespace Java.Interop {
 	partial class JniRuntime {
 
 		partial class CreationOptions {
+			public  bool                       UseMarshalMemberBuilder     {get; set;}      = true;
 			public  JniMarshalMemberBuilder    MarshalMemberBuilder        {get; set;}
 		}
 
@@ -24,6 +25,10 @@ public  JniMarshalMemberBuilder        MarshalMemberBuilder        {
 
 		partial void SetMarshalMemberBuilder (CreationOptions options)
 		{
+			if (!options.UseMarshalMemberBuilder) {
+				return;
+			}
+
 			if (options.MarshalMemberBuilder != null) {
 				marshalMemberBuilder    = SetRuntime (options.MarshalMemberBuilder);
 				return;