Support header and implementation jars in sdk

Add a new java_header_libs property that exports header jars (as
java_libs currently does) and switch java_libs to export implementation
jars instead.

Refactors implementation of the existing library sdk member type so
both properties can be supported from common code as they differ only
on the jar being exported.

Bug: 143678475
Test: m nothing
Change-Id: I04642122f72d083bbdfd3290624f957b71ee8875