Android: fixes some issues with JCamera.
Android: fixes some issues with JCamera.

- Quit the worker thread only when the worker is deleted. This makes
sure all events are processed before terminating the worker.
- Correctly cache the preview size even when it's an invalid one.

Task-number: QTBUG-36204
Change-Id: I76055984e8ece3f7f40dba7dd89d28a4faa1e72e
Reviewed-by: Denis Kormalev <8defd4f2cf6999201ff074728009327869e4f6c4@ics.com>
Reviewed-by: Christian Stromme <0d8a7c35440bf88bf05c30d94f2a26326f5cfba7@digia.com>

mer-qt/qtmultimedia,mer-qt/qtmultimedia,mer-qt/qtmultimedia,mer-qt/qtmultimedia,mer-qt/qtmultimedia