[native] update message list header to new design

Summary:
Updated the message list header based on how it looks in the figma design doc:
{F445548}

Android was already doing this, so I just needed to remove some conditional logic causing iOS to look different.

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Depends on D7132

Test Plan:
Please check out these screenshots to see the changes I made:

Before:

iOS:

Android:

After (pre avatars world):

iOS:

Android:

After (post-avatars world):

iOS:

Android:

Search:

iOS:

Android:

Reviewers: ashoat, atul

Reviewed By: ashoat

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D7134