[native] update selectedChannel state to be a FarcasterChannel type

Summary:
The selectedChannel state in the TagFarcasterChannel component was intially typed as a string. However, we should use the FarcasterChannel type here since I plan to use the farcaster channel id (which is a field in FarcasterChannel) in a subsequent diff

Depends on D11776

Test Plan: flow + confirmed that the existing farcaster channel selection user flow has no regressions

Reviewers: inka, atul

Reviewed By: inka

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D11784