Android asset extraction support

Signed-off-by: Filip Wasil <filip.wasil@tieto.com>
Any reason why this platform-specific logic should be coupled with the Importer? Could it not be implemented as a custom IOSystem (see Importer::SetIOHandler)? There are likely a number of platforms that have specific ways of accessing bundled or embedded assets, so we probably shouldn't make an exception for Android here. 

Perhaps we should consider an "extras" extension that provides useful (and optional) platform-specific constructs (like this), but in slightly more generic terms?