systemui: 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: I974f50806c724a400485de54589accbdde4bf70d
Signed-off-by: Jorge Ruesga <jorge@ruesga.com>
Merge remote-tracking branch 'cm/cm-11.0' into HEAD