[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
This message can be improved, especially because _assume_ here sounds like an instruction to the user, not a description of what is being done. Try something like _"--keystore-alias-passcode was not specified; using the keystore's passcode as the alias keystore passcode."_