Load QM files from assets: URLs on Android

Summary:
This works with both the old and the new way of Qt's asset deployment, ie.
with Qt 5.13 and Qt 5.14.

Reviewers: apol

Reviewed By: apol

Subscribers: apol, kde-frameworks-devel, kde-buildsystem

Tags: Frameworks, Build System

Maniphest Tasks: T12520

Differential Revision: https://phabricator.kde.org/D27596