settings: open add tile dialog

Shows the "add tile" dialog QuickSettingsTiles activity when the user press
the add tile button in the system bar

Change-Id: Id416b85aa11cee6fd586ee322650d421ca83cdf0
Signed-off-by: Jorge Ruesga <jorge@ruesga.com>
Merge pull request #6 from CyanogenMod/cm-11.0

settings: open add tile dialog