Check if the SubscriptionsChangeListener has been started before stop

calling mContext.unregisterReceiver on an unregistered broadcast
receiver will result in an exception. To make the
SubscriptionsChangeListener safe from this kind of error, add a check to
see whether start has been called before implementing stop's
functionality.

Bug: 184662284
Bug: 184521296
Test: atest -c SettingsUnitTests
Change-Id: Ic13911ee7573666f5d42ab4612c025968984ba47