[generator] Fix StackOverflow when copying DIM from package-protected interfaces

[jpobst]

Context: xamarin/AndroidX#988
Context: #1080

Imagine you bind the following Java interfaces:

package io.grpc;

public interface InternalConfigurator extends Configurator {}

interface Configurator {
  default void configureChannelBuilder(ManagedChannelBuilder<?> channelBuilder) {}
}

Because the Configurator interface is package-protected, its method is copied into the public InternalConfigurator interface via generator's FixupAccessModifiers code.

Later, we look for the "base" method that the copied InternalConfigurator.configureChannelBuilder overrides, which is Configurator.configureChannelBuilder. However, because we simply added the same method instance to two types, this method reference is actually pointing to itself.

Eventually we hit a StackOverflowException when trying to retrieve the overridden method's declaring type:

java-interop/tools/generator/SourceWriters/BoundMethod.cs

Lines 95 to 100 in 9d99723

	static string GetDeclaringTypeOfExplicitInterfaceMethod (Method method)
	{
	return method.OverriddenInterfaceMethod != null ?
	GetDeclaringTypeOfExplicitInterfaceMethod (method.OverriddenInterfaceMethod) :
	method.DeclaringType.FullName;
	}

Fix this by cloning the method when we "copy" it, rather than having two types reference the same Method instance.

[jpobst]

TODO from commit:

• Test this puppy in dotnet/android. Just because "it works" on Desktop JDK doesn't mean it does on Android.
• Update generator output to not emit the static readonly JniPeerMembers fields for internal types.

[jonpryor]

See also: dotnet/android#9397

[jonpryor]

"This puppy" has been tested on dotnet/android#9397, and is green. The approach works, and (equally important) the build system changes don't break things in dotnet/android.

We still need to "Update generator output to not emit the static readonly JniPeerMembers fields for internal types."

[jonpryor]

Draft commit message:

Context: https://github.com/xamarin/AndroidX/issues/988
Context: 9e0a4690adb71c04cd88a5b54bea3c3f8da73cc0
Context: https://github.com/dotnet/java-interop/issues/1269

Imagine you bind the following Java interfaces:

	// Java
	package io.grpc;

	interface Configurator {
	  default void configureChannelBuilder(ManagedChannelBuilder<?> channelBuilder) {}
	}

	public interface InternalConfigurator extends Configurator {
	}

Because the `Configurator` interface is package-protected, its method
is copied into the `public` `InternalConfigurator` interface via
`generator`'s `FixupAccessModifiers()` code.

Later, we look for the "base" method that the copied
`InternalConfigurator.configureChannelBuilder()` overrides, which is
`Configurator.configureChannelBuilder`.  However, because we simply
added the same method instance to two types, this method reference is
actually pointing to itself.  

Eventually we hit a StackOverflowException when trying to retrieve the
[overridden method's declaring type][0].

Fix this by _cloning_ the method when we "copy" it, rather than having
two types reference the same `Method` instance.

[0]: https://github.com/dotnet/java-interop/blob/9d997232f0ce3ca6a5f788b1cdb0fb9b2f978c84/tools/generator/SourceWriters/BoundMethod.cs#L95-L100