NEXUS-6125 adding apk, aar and apklib as default supported mime types

This allows for browsing aar and apklib files in the archive browser (they are just zip files) as well as sending a url to an apk in Nexus to a users, who can then download the apk (provided they have apk install activated). This can be used for Android application development e.g. to send an apk to qa in staging and so on..