[Android] Accept alias password for specified keystore when making apk

Password of alias and keystore can be different.
Previous make_apk.py only accepts keystore password.
Add additional option to accept alias password.

BUG=https://crosswalk-project.org/jira/browse/XWALK-2135
_Passcode of alias_ (here and in `make_apk`) is not very helpful. How about _"Passcode for alias's private key in the keystore"_?