[native] rename MessageResultsScreen to PinnedMessagesScreen

Summary:
In D10345 I got some feedback to update the MessageResultsScreen to PinnedMessagesScreen

Linear task: https://linear.app/comm/issue/ENG-6356/rename-messageresultsscreen-on-native-to-be-pinned-messages-screen

Test Plan: flow + confirmed PinnedMessagesScreen has no regressions

Reviewers: atul, inka

Reviewed By: atul

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D10983