Settings: Add the notification of connection change from wifi to mobile:

By default, android prefers to wifi if both wifi and mobile networks
are available. After wifi unavailable, android will automatically
switch to mobile.

Here a dialog is added to let user choose whether use mobile network
or not.

Change-Id: I219e0e964f8be2aad3da4ca93d0626cc3be6dd65
Signed-off-by: Xiaojing Zhang <zhangx@codeaurora.org>