Add basic Wikipedia Zero detection.

* If on Wikipedia Zero participating carrier, toast at startup.
* If NOT on Wikipedia Zero participating carrier, don't toast.
* Subsequent add or drop of network connection causes toast only when critical.
* This version uses uniform English ON / OFF verbiage.
* Future: Align on i18n approach.
* Future: iterate on how to show carrier baner TEXT from i18n system.
* Future: Design to style "W" slighlty different in W0 mode.
* Future: Intercept offsite links in W0 mode.
* Future: Show mini tutorial / dialog first 3 times (or less if pref changed) for offsite links in W0 mode. Shows FAQ link.
* Future: In-app user pref, default checked, to "Warn when leaving Wikipedia Zero". Shows current status plus FAQ link.

Change-Id: I6b8e952141e970a6a65b89756efaa125a91b5300