notif: Try to clarify "extract"-related logic a bit.
notif: Try to clarify "extract"-related logic a bit.

Add a bit of jsdoc; and refactor one caller to better separate dealing
with the platform-specific libraries vs. our application logic.

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