[Mono.Android] JavaList.Add should allow duplicates.

[jpobst]

Fixes: #9675

Remove the check for duplicates in JavaList.Add to allow duplicates.

[jonpryor]

Draft commit message:

Fixes: https://github.com/dotnet/android/issues/9675

Context: https://github.com/xamarin/monodroid/commit/0e782afc16fe4b7197c800864d6c10520905fe28
Context: https://github.com/xamarin/monodroid/commit/8d38265ba00678ff3f3dd6edd8ceb10a97b55abd
Context: https://github.com/xamarin/monodroid/commit/be4087f038957a36a0bb3b9f9dc401c65ef379ca

Binding is a historically hairy business, and sometimes we live with
the results of those choices decades later.

Case in point: what should be done about `java.util.List<E>`?
Some time around 2010-Sep-7 in xamarin/monodroid@0e782afc was a
"Generic and Collections restructuring", which resulted in the
*removal* of `java.util.List` and other interfaces from
`Mono.Android.dll`.  The idea was that bindings would instead use
`Android.Runtime.JavaList` and other types instead.

This was the continued state of affairs when [Bugzilla 8112][0] was
filed, which was an issue binding DroidText, which was simplified
into [Bugzilla 8134][1].  The underlying problem was that some type
in DroidText implemented `java.util.List`, which wasn't bound and
instead was replaced with `JavaList`, but the generated bindings
would attempt to override and implement `java.util.List` methods
such as `Add()` on `JavaList`, which didn't exist, resulting in
compilation errors.

The fix in 2012-Nov-2 in xamarin/monodroid@8d38265b was to add all
`java.util.List` methods onto `JavaList`, and as part of this was an
overlooked bug within `JavaList.Add(int, Object)`: it had a
`Contains()` check before trying to add!

	partial class JavaList {
	  public virtual bool Add (int index, Java.Lang.Object item)
	  {
	    if (Contains (item))
	      return false;
	    Add ((object) item);
	    return true;
	  }
	}

This in turn meant that duplicate values couldn't be added, even
though that is something that Java would allow.

The underlying problem of "`java.util.List` isn't bound!" was later
resolved in 2013-Mar-11 by xamarin/monodroid@be4087f0, which stopped
removing `java.util.List` from `Mono.Android.dll`.

There is another issue with the above `JavaList.Add()` method: it
ignores the `index` parameter!  Fix that as well.

[0]: https://web.archive.org/web/20210922182648/https://bugzilla.xamarin.com/81/8112/bug.html
[1]: https://web.archive.org/web/20210925214551/https://bugzilla.xamarin.com/81/8134/bug.html

[jonpryor]

@jpobst: there is another issue issue here: we have an Add(int index, Java.Lang.Object? item) method, but index isn't actually used!

Which means if someone tried:

list.Add ("foo");
list.Add ("baz");
list.Add (1, "bar");

the list contents would be { "foo", "baz", "bar" } not { "foo", "bar", "baz" }.

[jonpryor]

This new test is failing on CI:

Expected is <System.String>, actual is <Java.Lang.ICharSequenceInvoker>
Values differ at index [0]
Expected: 'A'
But was:  'C'

The SubList() test is also failing:

Expected is <System.String>, actual is <Java.Lang.ICharSequenceInvoker>
Values differ at index [0]
Expected: 'f'
But was:  'b'

Very odd. :-(

[jpobst]

Fixed.