Introduce droidcore-unbundled target

The droidcore-unbundled target replaces the droidcore
target when TARGET_BUILD_UNBUNDLED_IMAGE=true. This
target is similar to droidcore in terms of dependencies,
but it has a smaller set of dist files. The intention is
to use TARGET_BUILD_UNBUNDLED_IMAGE=true with a
VSDK-using vendor build to supply Java dependencies from
prebuilts to avoid building (or needing) Java framework
sources.

Test: build with and without TARGET_BUILD_UNBUNDLED_IMAGE=true
Test: m nothing dist (before patch)
Test: m nothing dist (after patch)
Test: m nothing dist TARGET_BUILD_UNBUNDLED_IMAGE=true (after patch)
Test: examine ninja build graph for previous 3 tests
Bug: 188176942
Bug: 187064252
Change-Id: Ia7f8200c1bb1aaaa57c177d1698a114efe26bec7