Compose the Android host into the root Gradle build

Two Gradle builds cannot share one settings file once their
dependencyResolutionManagement blocks disagree, and they do: the root build declares a
repository policy and the host application already declares its own. includeBuild
composes them instead, leaving both intact so the host still builds standalone.

This is the one non-additive step in the Android sequence, since it changes the root
build for everyone. It lands on its own so reverting is one click.

Two details confirmed by prototype: tasks in a composed build are not addressable
directly, so the root needs a delegate task per host; and that delegate names the included
build by its directory, not by its rootProject.name.

Depends on: #673.

Steps

• Compose the host build in from the root
• Add a root delegate task for the host
• Point the host's core dependency at the in-tree core
• Remove the pinned-core checkout step

Done when

The host builds from the repository root and still builds standalone, from a clean clone
with no machine-local configuration.