`AnimationGroup` with an introducer doesn't work when the mobject is already in the scene

Nobody has claimed this yet.

Consider the following scene:

```python
class Test(Scene):
def construct(self):
a = MathTex("a")
self.add(a)
self.play(AnimationGroup(Write(a)))
self.wait()
```

When run, instead of writing "a" like I would expect, the `Write` doesn't happen at all and the screen is blank for the first second. Making it not in an `AnimationGroup` fixes it, manually setting `introducer` to False fixes it, and removing `self.add(a)` fixes it.

Start by running the provided Scene reproducer and compare AnimationGroup(Write(a)) with the working cases that omit the group, omit self.add(a), or set introducer to False. Trace how AnimationGroup handles an introducer when its mobject is already in the scene; done when the grouped Write visibly animates the existing mobject.

Written by the indexing model from the issue text.