Port ExternalPartStorage::self to C++11 singleton syntax.

Summary:
Much simpler, and fixes a strange asan error about sInstance violating
ODR because defined twice in the test binary (which didn't seem right to me).

Test Plan: externalpartstoragetest now passes in the ASAN build

Reviewers: dvratil, vkrause

Reviewed By: vkrause

Subscribers: kde-pim

Differential Revision: https://phabricator.kde.org/D21141