stream settings: Fix "Private" switch behavior
stream settings: Fix "Private" switch behavior

The "Private" switch was stuck displaying its initial value. Use the
value being updated instead.

[Commit message amended by Ray Kraesig.]

Fixes: #3760

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