recipient [nfc]: Simplify pmUnreadsKeyFromMessage a bit.
recipient [nfc]: Simplify pmUnreadsKeyFromMessage a bit.

[This is a cherry-pick of d8f45122e, which was reverted in 084d726fc;
 see preceding commit.]

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