Android: camera moved to a dedicated thread

Actual camera work moved to JCameraWorker which lives in dedicated
thread. JCamera now acts as proxy calling JCameraWorker methods with
invokeMethod (when it is needed).

[ChangeLog][QtMultimedia][Android] camera operations moved to a
dedicated thread

Task-number: QTBUG-35564
Change-Id: Ie4edcbf0869d56b0fef4ad0c820450cc77657fdd
Reviewed-by: Yoann Lopes <yoann.lopes@digia.com>