DevTools: fix backend activation (#26779)

## Summary
We have a case:
1. Open components tab
2. Close Chrome / Firefox devtools window completely
3. Reopen browser devtools panel
4. Open components tab

Currently, in version 4.27.6, we cannot load the components tree.

This PR contains two changes:
- non-functional refactoring in
`react-devtools-shared/src/devtools/store.js`: removed some redundant
type castings.
- fixed backend manager logic (introduced in
https://github.com/facebook/react/pull/26615) to activate already
registered backends. Looks like frontend of devtools also depends on
`renderer-attached` event, without it component tree won't load.

## How did you test this change?
This fixes the case mentioned prior. Currently in 4.27.6 version it is
not working, we need to refresh the page to make it work.

I've tested this in several environments: chrome, firefox, standalone
with RN application.