Android: Make it possible to associate with files

Summary:
Put the file into a file descriptor and open this file descriptor.
Just application/pdf for now.

Test Plan: opened some documents on my phone

Reviewers: #okular, aacid, mart

Reviewed By: mart

Subscribers: okular-devel

Tags: #okular

Differential Revision: https://phabricator.kde.org/D12770