Settings: Mock SMS (3/4)

This change brings the possibility of simulate the reception of SMS on real devices.
This change includes the settings to enable/disable the mock SMS feature.

Ported from http://review.cyanogenmod.org/#/c/23018

Change-Id: I8818995147c41db261d9c9506c1741ed99ef581e
Signed-off-by: Jorge Ruesga <jorge@ruesga.com>