Simplify key-value mapping representation.

Intsead of an array of objects with keys "key" and "value", just use a
simple JSON object which encode key-value mappings more simply and
compactly.

Test: atest SignedConfigTest
Bug: 110509075
Change-Id: Ib81b6869d680de7b0e2c6b6b202dda2c7e854449