Unsubscribe React DevTools events

[EdmondChuiHW]

Summary

We should not have dangling listeners.

This could be the cause of some "Attempted to send a message to domain [] while queue is not empty" errors.

Stacked on #117

Test plan

1. Connect to DevTools
2. Use DevTools-on-DevTools to set a breakpoint in dispose.
3. Kill the native app and ensure dispose called (and runtimeModel is non-null`

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.

[hoxyq]

This could be the cause of some "Attempted to send a message to domain [] while queue is not empty" errors.

Oooh, great catch, thanks!