Encompassing one source pollsters with common context

After per-source separation of static resources & discovery samples are
publishing with context of separate pollster, so if it is used arithmetic
transformer which handle different meters, it will fail.

Now samples are publishing from common source context, after all
source meters have been collected.

Change-Id: I0d186247b4b87fd94317318c180bdfb9c6da70fc
Closes-bug: #1394228
Merge "Encompassing one source pollsters with common context"