Allow Android meterpreter to run as root

The Android meterpreter can run as root only if no android.* classes are loaded at Runtime. This change removes the dependency on android.content.Context from the android.payload.Stage interface, instead we use the original javapayload.stage.Stage interface and pass the writeable directory as a parameter.

I've also tested and merged the following PRs into this one:
https://github.com/rapid7/metasploit-javapayload/pull/14
https://github.com/rapid7/metasploit-javapayload/pull/17
https://github.com/rapid7/metasploit-javapayload/pull/22