Support zIndex Config for Transition

[qdsfdhvh]

To implement an animation such as the following, we need to use the zindex value.

[DevSrSouza]

It seems that this PR is doing more then just adding zIndex support, can you explaim in depth the changes and why they are needed and the use cases for it?

[qdsfdhvh]

I want to support animate like this, with zIndex:

untitled-ezgif.com-video-to-mp4-converter.1.mp4

without zIndex:

untitled2-ezgif.com-video-to-mp4-converter.mp4

AnimatedContent By default the targetState content will be at the top, so we need to configure the ZIndex to adjust the hierarchy;

contentAlignment: Alignment and contentKey: (Screen) -> Any are just the parameters of the AnmiationContent passed out, maybe it will be needed later;

For contentKey: (Screen) -> Any = { it.key } is just me that thinks { it.key } might be a bit more appropriate than the default {it}，also wo can back to the default value.

[DevSrSouza]

Can you explain what is this change are different from #445?

[qdsfdhvh]

The difference is still the same as enter/exit animation, #445 works for the default animation which needs to configure zIndex, but it's not convenient if it's a screen specific animation which needs to configure zIndex.