Add string resources for MD Ash network detailed view

Includes messages for when:
 - a network connection is connecting;
 - a network connection is connected;
 - a cellular network is not activated;
 - a VPN network may be monitored.
Also includes the title for cellular section.

BUG=none
TEST=

Committed: https://crrev.com/31b1215a3979bf9cee3206141a1970bb84afb775
Review-Url: https://codereview.chromium.org/2478713002
Cr-Original-Commit-Position: refs/heads/master@{#429750}
Cr-Commit-Position: refs/heads/master@{#429838}