settings: Make mobile settings a first-class citizen

 * Nobody likes carriers, but let's not bury these settings behind
   10 layers of indirection. Many users (like me) who travel often
   will need to access these options frequently.
 * The top level switch disables mobile data.

Change-Id: I1bd4d1116a2c73832230d66c94df2c3c0aacf698
(cherry picked from commit 3b89d5bd97e57403d869a032abc0eab23aa7cf9d)