…ew instance if the existing one is of the wrong type. Fixes #13704. (#16502)

Objective-C has an optimization where creating an empty dictionary would
return the same instance every time (probably to a constant empty
dictionary).

This causes a problem with how we've bound generic dictionaries, because
all
empty dictionaries would have the same native handle, even if we'd bound
them
with different managed types.

This surfaces in unfortunate ways when we try to look up a managed
instance
given a native handle, we find that we already have a managed instance,
but
the managed instance is of the wrong type.

Example code:

    var a = new NSDictionary ();
    var b = new NSDictionary<NSString, NSString> ();
    var c = new NSDictionary<NSString, NSObject> ();
    Console.WriteLine ($"a: 0x{a.Handle:X}");
    Console.WriteLine ($"b: 0x{b.Handle:X}");
    Console.WriteLine ($"c: 0x{c.Handle:X}");

would produce something like:

    a: 0x0x7fff80821080
    b: 0x0x7fff80821080
    c: 0x0x7fff80821080

now for this code:

    Runtime.GetNSObject<NSDictionary<NSString, NSString>> (b.Handle)

it would throw an exception like this:

Unable to cast object of type
'Foundation.NSDictionary`2[[Foundation.NSString],[Foundation.NSObject]]'
to type
'Foundation.NSDictionary`2[[Foundation.NSString],[Foundation.NSString]]'

because we'll have the 'c' object (with type `NSDictionary<NSString,
Object>`)
in our dictionary of native handles -> managed instances, and that's not
compatible with the desired return type from GetNSObject
(`NSDictionary<NSString, NSString>`)

This likely happens with all the non-mutable collection types we have a
generic version of (NSArray, NSDictionary, NSOrderedSet, NSSet).

Fixes #16378.
Fixes #13704.

This PR is somewhat simpler to review by ignoring whitespace:
https://github.com/xamarin/xamarin-macios/pull/16491/files?w=1


Backport of #16491

Co-authored-by: Rolf Bjarne Kvinge <[email protected]>