Parallelize system*.img and target-files-package

We don't actually need to build the system image before
target-files-package, we just need to make sure everything is present in
$(PRODUCT_OUT)/system. So switch the dependencies around.

This can't be done as easily for the vendor image, since it may be a
prebuilt, or we may need to install some kernel modules into
$(PRODUCT_OUT)/vendor when building vendor.img. It tends to be smaller
anyway, so the time savings isn't as big.

Test: Make sure the build server outputs the same files
Test: Ensure target-files-package is identical before/after.
Change-Id: I84e4f878491ab2b71582f4f49418dbfbba75590f