compose box: Inline some logic into handleMessageFocus.
compose box: Inline some logic into handleMessageFocus.

This is a pure refactor: we just expand what setTopicInputValue does,
and in turn its callee handleTopicChange.

The expanded version makes it obvious there's some redundancy.
We'll clean that up separately.

vishwesh3/zulip-mobile,vishwesh3/zulip-mobile,vishwesh3/zulip-mobile,vishwesh3/zulip-mobile