StreamItem: Switch to new React Context API for theme colors.
StreamItem: Switch to new React Context API for theme colors.

Per #1946, we're moving to the new React Context API
(https://reactjs.org/docs/context.html).

Use the same strategy as in the parent commit, for SmartUrlInput.

Also, remove what looks like an ill-considered use of
StyleSheet.flatten, which takes an array of objects:

> Flattens an array of style objects, into one aggregated style
> object. Alternatively, this method can be used to lookup IDs,
> returned by StyleSheet.register.

(https://reactnative.dev/docs/stylesheet#flatten)

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