OTA Tools: Handle password encrypted keys

* Add logic to handle decrypted keys from common.GetKeyPassword in
WriteABOTAPackageWithBrilloScript.

* Get the keys passwords in main and store them in OPTIONS.key_passwords.
This allows accessing them in WriteABOTAPackageWithBrilloScript and SignOutput
so it's only required to ask for the password once, while allowing to use
decrypted signing keys.

Test: ota_from_target_files.py on marlin and angler respectively.
Change-Id: I7c9b0198855a4b630c52b8552e904f312f09c4ce