Simplify appearance of internal feedback button

[huntie]

Summary

Tweaks our custom-styled internal feedback toolbar item. Reduces visual prominence so that other items / future background colouring will fit better.

Test plan

• This change maintains backwards compatibility with previous Local Storage data (if modifying settings, experiments, or other persisted client state).

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.