[Tizen] Drop privileges for extension process

This commit uses security-manager API to apply correct security context
for extension process.

BUG=XWALK-1731

[Verification]
  rpm -i security-manager-0.1.0-2.2.armv7l.rpm \
         libcynara-admin-0.2.2-10.1.armv7l.rpm \
         cynara-0.2.2-10.1.armv7l.rpm \
         libsecurity-manager-client-0.1.0-2.2.armv7l.rpm
  systemctl start cynara
  systemctl start security-manager
  systemctl enable security-manager
  (install and run app)

Depends on: https://github.com/crosswalk-project/crosswalk/pull/2518
That being said, I don't think that this part should be included in this patch. It's a part of a bigger solution for passing capabilities to launchers on Tizen. You included only part of it, which is not enough. And it's still under discussion and review and should not be included here.