ART: Add some utilities for working with containers.

Add utility functions for searching, removing and replacing
existing values in a container, to be used with std::vector
(including the ArenaVector alias) and other containers.

Also move UniqueCPtr<> and MakeUnique() to base/stl_utils.h
and clean up related includes.

Change-Id: I1e61762df91c046866591bda167d42bf8b67a692