[native] Add some memoization to InnerRobotextMessage

Summary:

1. Memoizing textParts will prevent their parent Text from rerendering when unnecessary
2. Wrapping the whole component with React.memo brings it back to parity with the old PureComponent implementation

Test Plan: Flow, making sure robotext messages still render on native

Reviewers: atul

Reviewed By: atul

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D6504