[RN0.66] 40/n: Fix FlatList type in ChatList

Summary: This type error was caught as part of the Flow upgrade. Managed to remove a $FlowFixMe too!

Test Plan: Flow

Reviewers: palys-swm, atul

Reviewed By: palys-swm

Subscribers: benschac, Adrian, karol-bisztyga

Differential Revision: https://phabricator.ashoat.com/D3021