Refactor internal methods to avoid duplicate JVM signature

[jbarr21]

ViewRouter extends from Router & both have a method: internal fun onSaveInstanceStateInternal(outState: Bundle).
In order to avoid exposing this method to non-RIB-class subclasses, the method was duplicated each Router class w/ internal visibility. But, starting in K2 this fails with duplicate JVM signature, so this PR removes the ViewRouter copy & makes the Router copy public but requiring a core annotation.

[psteiger]

I propose a different approach:

1. Completely kill ViewRouter.saveInstanceStateInternal -- it seems the only reason it existed was to force a friend-module visibility by duplicating the method name in a class in the module that will access it -- kinda hacky.
2. Make Router.saveInstanceStateInternal public, but annotate it with @CoreFriendModuleApi.

RibActivity now can access Router.saveInstanceStateInternal because rib-android is a core-friend module (opt-in from Gradle)

Now, external Kotlin consumers can't just @OptIn(CoreFriendModuleApi::class) because it is an internal annotation. So it remains inaccessible to them.

External Java consumers can access it, but that's the same issue with just internal visibility anyway -- it's public but mangled.

See #618

[psteiger]

Let me know what you think of the suggested approach -- I think it's being pushed by JetBrains own libraries to handle the "internal to friend modules" visibility issue and I'm pushing for the same here.

[jbarr21]

I like it, thanks for sharing. Definitely feels better, too