MountService listeners can not be unregistered

When unregistering MountService listeners, their Binder proxy
objects were compared to registered listeners instead of their
backing objects.

Change-Id: Ie98a686b1d17544f1eda59fb3eb0dc02d836c1b2
Add NULL check in updateProxyConfig

If the exclusionList was null in updateProxyConfig the VM would
crash when converting it to a UTF8 string. Avoid this by adding a
null check.

Change-Id: I0d8106fd54385bd9ae9c652a6c67d459a119cf2b