avoid crashing system serer process

Context#unregisterReceiver will throw IllegalArgumentException
if the receiver has already unregistered.

In GlobalAction class, when SilentModeAction is changed, it will
close the dialog with some delay. So there is a possibility
that it would unregistere again for the same receiver if a user
changes slient mode againand again. System must not crash in such
a case.

Change-Id: I33662feb48a770d39a8413901ec77ac1acd16bc7