[Mono.Android] Java.Lang.Throwable overrides Exception.ToString()

Fixes: https://bugzilla.xamarin.com/show_bug.cgi?id=42366

A funny thing happened between Xamarin.Android 6.0 and 6.1: we bumped
from Mono 4.2 to Mono 4.4, and Mono 4.4 now uses Microsoft's
referencesource for `System.Exception`.

In Mono 4.2, `System.Exception.ToString()` accesses the
`System.Exception.StackTrace` property. Since
`Java.Lang.Throwable` overrode the `Exception.StackTrace` property to
append the Java-side stack trace, this was sufficient to cause
`.ToString()` usage to likewise contain the java-side stack:

	var e = new Java.Lang.Throwable ("Boo!");
	var s = e.ToString ();
	// s contains "... --- End of managed exception stack trace --- ..."

This is *not* the case in Mono 4.4: `Exception.ToString()` *doesn't*
use the `Exception.StackTrace` property, so the `Throwable.StackTrace`
override is ignored, making for less-than-useful `.ToString()` value:

	// s == "Java.Lang.Throwable: Exception of type 'Java.Lang.Throwable' was thrown"
	// s does NOT contain the Java-side callstack!

Update `Java.Lang.Throwable` to override `Exception.ToString()` so
that the Java-side stack trace is always appended, if present. This
allows `Exception.ToString()` to be far more useful:

	Java.Lang.Throwable: Exception of type 'Java.Lang.Throwable' was thrown.
	  --- End of managed Java.Lang.Throwable stack trace ---
	java.lang.Throwable: Boo!
	  at md5bb7ab8908f0a1e03a3f881aa9f8f5eef.MainActivity.n_onCreate(Native Method)
	  at md5bb7ab8908f0a1e03a3f881aa9f8f5eef.MainActivity.onCreate(MainActivity.java:29)
	  ...

Author: jonpryor

src/Mono.Android/Java.Lang/Throwable.cs

@@ -13,7 +13,6 @@ public partial class Throwable : global::System.Exception, IJavaObject, IDisposa
 #endif  // JAVA_INTEROP
 	{
 
-		static IntPtr java_lang_Object_toString;
 		protected bool is_generated;
 		IntPtr handle;
 
@@ -130,20 +129,45 @@ IntPtr IJavaObjectEx.ToLocalJniHandle ()
 
 		public override string StackTrace {
 			get {
-				if (string.IsNullOrEmpty (nativeStack) && handle != IntPtr.Zero) {
-					using (var nativeStackWriter = new Java.IO.StringWriter ())
-					using (var nativeStackPw = new Java.IO.PrintWriter (nativeStackWriter)) {
-						PrintStackTrace (nativeStackPw);
-						nativeStack = nativeStackWriter.ToString ();
-					}
-					nativeStack = base.StackTrace + Environment.NewLine +
-						"  --- End of managed exception stack trace ---" + Environment.NewLine +
-						nativeStack;
+				return base.StackTrace + ManagedStackTraceAddendum;
+			}
+		}
+
+		string ManagedStackTraceAddendum {
+			get {
+				var javaStack = JavaStackTrace;
+				if (string.IsNullOrEmpty (javaStack))
+					return "";
+				return Environment.NewLine +
+					"  --- End of managed " +
+					GetType ().FullName +
+					" stack trace ---" + Environment.NewLine +
+					javaStack;
+			}
+		}
+
+		string JavaStackTrace {
+			get {
+				if (!string.IsNullOrEmpty (nativeStack))
+					return nativeStack;
+
+				if (handle == IntPtr.Zero)
+					return null;
+
+				using (var nativeStackWriter = new Java.IO.StringWriter ())
+				using (var nativeStackPw = new Java.IO.PrintWriter (nativeStackWriter)) {
+					PrintStackTrace (nativeStackPw);
+					nativeStack = nativeStackWriter.ToString ();
 				}
 				return nativeStack;
 			}
 		}
 
+		public override string ToString ()
+		{
+			return base.ToString () + ManagedStackTraceAddendum;
+		}
+
 #if JAVA_INTEROP
 		public int JniIdentityHashCode {
 			get {return (int) key_handle;}