keystone: require an explicit android_target for container.py

Before this change, container.py would use the first entry of the
OVERLAY_MAP dict as the default android target. However, python dicts
have no defined ordering, so the default is subject to change based on
how the dict is created, what version of python is in use, etc. Instead,
require that consumers specify their android target explicitly.

Bug: 118129494
Change-Id: Ife01a8495cac04670ea18468436c46a950fa1f5b