Description

Convert SignatureRequest into functional component:

https://github.com/MetaMask/metamask-mobile/blob/18dc20b8cc9f3f0707f5eb04f2ac88ee320c93bb/app/components/UI/SignatureRequest/index.js

Acceptance Criteria

• Signature request functionality continues to work as it is currently
• Use of deprecated react api is removed

References

https://docs.google.com/document/d/19xKY4e07UThOa1jsDQA8pjsNLUBRn39EdUbaFpz3OR0/edit?usp=sharing