Revert "fix(magnification thumbnail): remove feature flag, enable thumbnail by default"

This reverts commit 564f3448.

Reason for revert: We are going to add a proper setting and a new-style flag for this https://buganizer.corp.google.com/issues/293863788

Change-Id: I3f580d93d0059557d905168b2dbb88c2d748af3b