improve password entry for signing keys

Allow the user to set ANDROID_PW_FILE to the name of a file for
storing password keys.  When the tools need additional passwords, they
will rewrite this file and invoke the user's editor for the new
passwords to be added.  This allows passwords to be reused across
invocations of the signing tools, without making the user reenter them
every time.

Paranoid users can use a file stored in a ramdisk, or not use this
feature at all (the code will prompt for passwords in the ordinary way
when ANDROID_PW_FILE is not set).