Add ContentObserver when sms sent limit changed

When SMS message limit is changed in settings, it will only takes
effect after restart the phone.
Add ContentObserver to listen for SMS message limit changes.

FEIJ-716

Change-Id: I1a2294f5b3b1e3e55692e563cefbaa154cdb7194
Signed-off-by: Joey <joey@lineageos.org>