Add survey setting: Data collected by others visible / invisible

[jo-spek]

Data collected by others have previously been visible to data collectors and survey organizers in Ground for Android. That has been deactivated in response to #1800 for privacy reasons.
However, in the current Ghana campaign the organizers would specifically like data collectors to see each others collected data to avoid duplicates.

It would therefore be great to implement a setting in survey management to switch on/off the visibility of others' data in Ground for Android.

(Additional info: Survey organizers obviously can see data collected by others in Ground-platform, but NOT in Ground for Android!)