Add support for creating APKs from Craft's image directories

Unlike using the Craft root directory this will present only the relevant subset of files to androiddeployqt, allowing us to use the the much more flexible exclusion lists from Craft for the APKs as well.

Technically this is done here by allowing to specify a staging root dir that takes precedence over all build and install paths when set. All content is then taken from there rather than the folders needed/used for building.