Heads up master switch (2/2)

Add an short explanation abaout how to enable Heads Up per app for those who don't like to read or serch in changelogs

Change-Id: I2f00ad71e564bc15c3904d295cfdccc4a4673996
Add more explanation in case of disable heads up through master switch

- Call ui and mms will stop receive heads up notifications automatically

Change-Id: Ie800b84e203d59cb922b82815f538cc28ec1c8e6