Make framework-doc-stubs use jarstub defaults

These stubs used to duplicate all the properties in the defaults
we had already defined, but did not set e.g. sdk_version: "none",
which meant certain symbols were imported twice (which is error
prone / wrong as they could potentially be different).

Test: m framework-doc-stubs before & after and diff soong intermediates
Change-Id: I96dfa66a9deea3564980678fd64f2bfc6460eebd