Add support for using custom signapk.jar.

Details:
* New --signapk_path, --extra_signapk_args, --java_path.
* New --public_key_suffix, --private_key_suffix so you can change the filenames.
* Fixes raising exceptions on error.

Change-Id: I0b7014b6d779d52ae896f95dfecb1bcccf536cf4