[animations] Name conflict for DualTransitionBuilder

[shihaohong]

Since DualTransitionBuilder was introduced to flutter/flutter: #58686, this can result in a naming conflict for the animations package, since the animations package also contains an identical DualTransitionBuilder.

We should have the animations package use its own copy of the widget until DualTransitionBuilder reaches the stable branch. Then, we can safely remove the copy of DualTransitionBuilder in the animations package.

[stuartmorgan-g]

Presumably this follow-up step could happen now?