fix[ReactDevToolsViewBase]: observe model from constructor

[hoxyq]

Summary

Stacked on #111. See this commit.

We can observe the model from the constructor, so that it is called only once per panel. This pattern is used across the project.

Test plan

Manually test that React DevTools panels work correctly.

Upstreaming plan

• This commit should be sent as a patch to the upstream devtools-frontend repo. I've reviewed the contribution guide.
• This commit is React Native-specific and cannot be upstreamed.