[Mono.Android] Fix incorrect Context.RegisterReceiver enumification.

[jpobst]

Fixes #7503

Android.Content.Context.RegisterReceiver(BroadcastReceiver, IntentFilter, int) (+1 overload) was incorrectly enumified as ActivityFlags when it should have been ReceiverFlags.

To fix without breaking API:

• Create new overloads methods that chain to the existing incorrectly enumified methods.
• Mark the existing methods as [Obsolete] with description.

Additionally, it was noticed that the public API tracking added in #8171 did not get flagged as an error, only a warning. Change to an error to ensure any new public API is accounted for.

[jonpryor]

What gives me pause is that the resulting binding makes an abstract method [Obsolete], a'la:

partial class Context {
    [Obsolete ("This method has an incorrect enumeration type. Use the overload that takes ReceiverFlags instead.")]
    public abstract Android.Content.Intent? RegisterReceiver (Android.Content.BroadcastReceiver? receiver, Android.Content.IntentFilter? filter, [global::Android.Runtime.GeneratedEnum] Android.Content.ActivityFlags flags);
}

which I believe means that every override also needs to be [Obsolete], hence the changes in this PR to ContextWrapper and MockContext. This feels "weird".

On the plus side, (effectively) nobody should ever override Context.RegisterReceiver(), so this should be safe.

[jpobst]

This feels "weird".

Agreed.

On the plus side, (effectively) nobody should ever override Context.RegisterReceiver(), so this should be safe.

This is the rationale I eventually went with. ~No one should override this, users should only be calling the system provided version(s) of it.