SignApk - use existing password mechanism when using keystore

Summary:
In my last diff, I've added mechanism to load private key from keystore.
However, that mechanism will reveal password as part of the java param.
This diff tries to use existing ANDROID_PW_FILE mechanism to support
password for keystore private keys (through stdin)

This diff also fix a null pointer bug in the existing password handling

Test: This diff has been tested locally, and could sign correctly with our
  keystore with or without password

Tags:
Change-Id: Ie291ea8702a3b4d270b0f8689b023c3f290980a7