Simplify declaration and generation of unique_ptrs

Simplify declaration and generation of unique_ptrs

The unique_ptr-based RAII pattern is used in several places in
security-manager. Declaration of unique pointer variables can be awkward
and hard to read.

This patch hides the nasty details of unique_ptr types declaration behind
a template function. It is loosely inspired by std::make_unique from C++14.

Change-Id: Ifbd8b5ab409fd8646d149d6294cb60bd2ac873a8