[Java.Interop] public ctor for JniRuntime.JniObjectReferenceManager (#527)

Context: https://devdiv.visualstudio.com/DevDiv/_build/results?buildId=3273185&view=logs&jobId=b294c323-e432-5e2d-7a4a-9afaf424764a&j=b294c323-e432-5e2d-7a4a-9afaf424764a&t=296c62e6-5986-5511-382c-2bc427b1c780

Commit 5fa8c18 removed the public constructor from the abstract type
`Java.Interop.JniRuntime.JniObjectReferenceManager`.  This change is
fine, but it triggered the API Compatibility break:

        <div data-is-breaking>	<span class='removed removed-inline removed-breaking-inline'>public</span> <span class='added '>protected</span> JniRuntime.JniObjectReferenceManager ()

As we are in the middle of [replacing the API compat checks][0],
it's easier to simply revert this change for now.

[0]: dotnet/android#3884

Author: jpobst

src/Java.Interop/GlobalSuppressions.cs

@@ -8,6 +8,8 @@
 
 [assembly: SuppressMessage ("Design", "CA1008:Enums should have zero value", Justification = "No thanks", Scope = "type", Target = "~T:Java.Interop.JniVersion")]
 
+[assembly: SuppressMessage ("Design", "CA1012:Abstract types should not have constructors", Justification = "Public API checker in flux - Can change later", Scope = "type", Target = "~T:Java.Interop.JniRuntime.JniObjectReferenceManager")]
+
 [assembly: SuppressMessage ("Design", "CA1030:Use events where appropriate", Justification = "This isn't 'raising' an event; it's 'raising' a pending exception within the JVM.", Scope = "member", Target = "~M:Java.Interop.JniRuntime.RaisePendingException(System.Exception)")]
 
 [assembly: SuppressMessage ("Design", "CA1024:Use properties where appropriate", Justification = "<Pending>", Scope = "member", Target = "~M:Java.Interop.JniRuntime.GetRegisteredRuntimes()")]

src/Java.Interop/Java.Interop/JniRuntime.JniObjectReferenceManager.cs

@@ -14,6 +14,10 @@ partial class JniRuntime {
 
 		public abstract class JniObjectReferenceManager : IDisposable, ISetRuntime {
 
+			public JniObjectReferenceManager ()
+			{
+			}
+
 			public  JniRuntime      Runtime { get; private set; }
 
 			public virtual void OnSetRuntime (JniRuntime runtime)