[android] What is Android keystore file, and what is it used for?

The whole idea of a keytool is to sign your apk with a unique identifier indicating the source of that apk. A keystore file (from what I understand) is used for debuging so your apk has the functionality of a keytool without signing your apk for production. So yes, for debugging purposes you should be able to sign multiple apk's with a single keystore. But understand that, upon pushing to production you'll need unique keytools as identifiers for each apk you create.