Tweak JSON-LD support to generate more idiomatic JSON-LD

Lean on the @context more to generate shortnames for fields and
hoist @type declarations so that the actual node objects are as
concise as possible. These structures are semantically equivalent
to the previous markup, but should download quicker and be easier
for consumers to use.

Bug: T44063
Bug: T164655
Change-Id: I45294572607926b68c60c9c2c0f306b8c8d14bc3