Fix permissions handling.

+ Rename the permissions as per: http://go/android-multinetwork-routing
+ Make the SYSTEM permission explicitly include NETWORK.
+ Grant the SYSTEM permission to system UIDs by default, but allow the framework
  to override them if necessary.
+ Move the "string to permission" parsing to CommandListener.cpp, thus allowing
  us to get rid of Permission.cpp.
+ There's no need to support multiple permissions string arguments, so tighten
  that up.

Change-Id: I73d51b5e2f44a97e6d5ab5943ff198cebfbcc0c4