streamItem: Improve logic to determine icon color.
streamItem: Improve logic to determine icon color.

If color is not undefined then use it else decide it from
background. As if someone is explicitly specifying color
then it is expecting it to be used, rather than computing.

Fix: stream icon color is black in all stream screen in dark theme.

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