[lib] Make sure bottom-right corner of pending sidebar message is rounded

Summary:
The danger of manually generating ChatMessageInfoItem (as we do in getSourceMessageChatItemForPendingSidebar) is that it becomes the developer's responsibility to set startsConversation, startsCluster, and endsCluster correctly.

This was done incorrectly in D579, leading to the pending sidebar message not having its bottom-right corner be rounded. This should've been caught during testing & review of D579. Going forward I guess I'll need to start asking for screenshots of every UI diff.

Test Plan: Make sure the corners are correctly rounded

Reviewers: atul, KatPo, palys-swm

Subscribers: zrebcu411, Adrian, subnub

Differential Revision: https://phabricator.ashoat.com/D866