Zip after add_img_to_target_files

For aosp_marlin (on AOSP), this speeds up target files creation from
~57 seconds to ~15 seconds.

For marlin-userdebug (on internal master), this speeds up target files
creation from ~300 seconds to ~95 seconds.

This does make some minor changes to the resulting target_files zip:

* All of the META files are actually at the beginning of the zip now,
  previously the ones created by add_img_to_target_files would be at
  the end.
* The images are slightly larger. Go's zip implementation in soong_zip
  isn't quite as good as the `zip` tool.
* vendor_filesystem_config.txt isn't present if we're not building a
  vendor image.

Test: aosp_marlin target_files zip changes look reasonable
Test: marlin target_files zip changes look reasonable
Test: bullhead target_files zip changes look reasonable
Change-Id: Ia5ce6870b85559d65107c3b94332391b4b43a0ea