Re-home the Android host workflows

Workflows under a subdirectory are ignored, because only the repository root is read. Each
of the host's workflows is therefore re-created at the root and gated on the host's change
output. Local composite actions can stay where they are and be referenced by their new
path.

Depends on: #673, #670, #672.

Steps

• Re-create the lint and unit-test workflows at the root, gated
• Re-create the instrumented-test workflow, gated
• Re-create the distribution workflows, gated
• Reference composite actions by their new path

Done when

A host-only pull request runs the host's checks, and a docs-only pull request runs none of
them.