[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
The proposed form is a common practice in spec files (see Fedora recommendations for example: http://fedoraproject.org/wiki/Packaging:ScriptletSnippets#Shared_libraries). Dependency on a specific binary is well handled by rpm and is needed because of the line 284, where the said binary is used. The purpose of `%{_sbindir}` is completely different - it's for placing files in the package.