notifications: Refactor update_notification_sound_source.
notifications: Refactor update_notification_sound_source.

The container element and settings object are passed as
parameters to update_notification_sound_source. This change
is done so that we can use the same function for updating
the realm-level notification sound source.

zulip/zulip,rht/zulip,kou/zulip,rht/zulip,andersk/zulip,rht/zulip,zulip/zulip,eeshangarg/zulip,rht/zulip,zulip/zulip,eeshangarg/zulip,kou/zulip,zulip/zulip,zulip/zulip,eeshangarg/zulip,rht/zulip,andersk/zulip,zulip/zulip,eeshangarg/zulip,kou/zulip,kou/zulip,eeshangarg/zulip,rht/zulip,andersk/zulip,andersk/zulip,andersk/zulip,zulip/zulip,andersk/zulip,kou/zulip,eeshangarg/zulip,kou/zulip,rht/zulip,eeshangarg/zulip,andersk/zulip,kou/zulip