Server certificate trust, code review fixes

Added empty certificate trust store.
Now server certificate trust checks follow this order:
1) If "res/raw/client_truststore.bks" is not empty, use it as the
   pinned cert trust store (default is empty)
2) Otherwise, if the "Show certificate dialog" developer preference
   is enabled, use that (default is disabled)
3) Otherwise, use the default system trust store, consists of
   normal trusted Android CA certs
Also fixed typo in comment in SVMPAppRTCClient.
Note: the empty trust store can be replaced with one of your
choosing. The password to access that trust store should be set
accordingly in Constants.java.

Change-Id: Ifa799a7e858ff5f0d2413568236c19a169206a05