Add TARGET_COPY_OUT_* variables.

They are relative to PRODUCT_OUT so can be used in PRODUCT_COPY_FILES.
If TARGET_COPY_OUT_VENDOR instead of hardcoded system/vendor is used, we
will be able to transition smoothly to a separate vendor staging directory.

Change-Id: Ia9fdb208768cf4699180306280b4c02421b2ea9c