add android_filesystem

android_filesystem is a module type that can be used to create filesystem
images out of binaries built for Android. Its initial use will be for
creating an Android-like OS image to run on virtual machines, but the
use is not limited to it.

The module type currently lacks a lot of features like the support for
other filesystem types, and the ability to do something like signing the
image using avbtool, etc. Those will be added in follow-up CLs as we
have actual demands.

Bug: 172414391
Test: m
Change-Id: I4b779d4586e04d9a960688e73b711166708558ab