Android: scan for qml files in the source dir, not in the install dir.

Summary:
When the QML files are all bundled into a .qrc file, they don't get
copied to the install dir, which would lead to qmlimportscanner not picking
up the dependencies (e.g. QtQuick Controls) for android packaging.

Looking at commit 1b0496d, I wonder if maybe we should be able to specify
two paths to look into? But qmlimportscanner doesn't support that, does it?

Test Plan:
using this toolchain for a basic QtQuick2 app created from a
QtCreator template, with Qt Quick Controls support, and QML files
in a Qt resource.

Reviewers: apol, mart

Reviewed By: apol

Subscribers: Build System, Frameworks

Tags: Frameworks, Build System

Differential Revision: https://phabricator.kde.org/D6466