[Android] Passwords of keystore as argument should not be mandatory

When make apk, if user wants to use their own keystore, they must
provide the password as command line argument for now.
Developers may have security concern for this.

This commit allows developers to input password as hidden characters
while making apk.

Thus, if developer still specifies password in command line, the
"ant.properties" file which carries the password will be removed
after build.

BUG=https://crosswalk-project.org/jira/browse/XWALK-2138