Fetch generated srcs from java libs for droiddoc.

api-stubs, system-api-stubs and etc need generated sources and srcjars from "framework",
so add a property that tell module to fetch srcs and srcjars from its
dependency libraries. The libraries in that property has to be in the
module's classpath.

Also add doc_defaults targets.

Bug: b/70351683
Test: m -j
Change-Id: I05831fbcad488037710950e4f05dc8fb2a12f403