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

[jonpryor]

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)
  ...

[garuma]

👍