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