Android: Release the surface texture when not in use

With some Android versions the preview texture is released when the
application is suspended. If we don't release the texture in our
code, the preview will be empty when the application resumes.

Task-number: QTBUG-38165

Change-Id: I72244727081d8f94ee5f6cb0ab660ca59f4bb2de
Reviewed-by: Yoann Lopes <yoann.lopes@digia.com>