dillon1000/react

refactor[Agent/Store]: Store to send messages only after Agent is initialized (#30945)

Both for browser extension, and for React Native (as part of
`react-devtools-core`) `Store` is initialized before the Backend (and
`Agent` as a part of it):

https://github.com/facebook/react/blob/bac33d1f82d9094b45d6f662dd7fa895abab8bce/packages/react-devtools-extensions/src/main/index.js#L111-L113

Any messages that we send from `Store`'s constructor are ignored,
because there is nothing on the other end yet. With these changes,
`Agent` will send `backendInitialized` message to `Store`, after which
`getBackendVersion` and other events will be sent.

Note that `isBackendStorageAPISupported` and `isSynchronousXHRSupported`
are still sent from `Agent`'s constructor, because we don't explicitly
ask for it from `Store`, but these are used.

This the pre-requisite for fetching settings and unsupported renderers
reliably from the Frontend.