[Extensions][Tizen] Change ExtensionProcess SMACK label.

In order to keep its privileges to a bare minimum, the
ExtensionProcess (EP) needs to:

1- Change its SMACK label from "xwalk" to its web app id;
2- Drop the CAP_MAC_ADMIN capability.

The first step will guarantee that the EP not only
runs with a different SMACK label from the runtime (xwalk) but it
will also make EP run with a label that provides access to all
the capabilities needed by its web app.

The second step drops the capability that enables a process
to change its SMACK label and execute other MAC related commands.