Enable RememberObserver to work with rememberRetained

[chrisbanes]

This PR manually calls RememberObserver calls as appropriate when used in rememberRetained. The semantics of rememberRetained are obviously different to remember, so we call them at different times:

• onRemembered will be called the first time that the retained item is first remembered. The difference here is that we will not call onRemembered again once a the object is restored and remembered back into composition.
• onForgotten will be called only once the retained state is cleared from both composition and any retained registries.
• Thus, the logical lifecycle is maintained of created (onRemembered called), through to being no longer retained/remembered (onForgotten called).
• We do not call onAbandoned as retained state doesn't meet the contract of the function.

I did think about creating a RetainedObserver interface, but figured that using the existing RememberObserver, even with slightly different semantics, is a better solution.

This has a variety of use-cases, but I have been playing around with a rememberRetainedCorotineScope in Tivi: chrisbanes/tivi#1763

[ZacSweers]

Nice find

[chrisbanes]

Ready for review @ZacSweers!