Replace `mountDepth` with `isTopLevel`

Summary:
After #2570, `mountDepth` is only used to enforce that `setProps` and `replaceProps` is only invoked on the top-level component. This replaces `mountDepth` with a simpl\
er `isTopLevel` boolean set by `ReactMount` which reduces the surface area of the internal API and removes the need to thread `mountDepth` throughout React core.

Reviewers: @sebmarkbage @zpao

Test Plan:
Ran unit tests successfully:

```
npm run jest
```
Is it safe here to assume that `this` is going to be a `ReactCompositeComponent`?