Fix launching Okular by opening a document from the file browser

Summary:
There's two issues here:

• Activity::onCreate triggered before the C++ side was loaded, so the call to C++ failed (causing a crash). So delay that until we are ready.
• The static instance of URIHandler in the header caused it to duplicate into each translation unit, causing main.cpp to not see what android.cpp wrote into it.

Subscribers: okular-devel

Tags: Okular

Differential Revision: https://phabricator.kde.org/D18399