Adding vbmeta_system.img and vbmeta_vendor.img to droidcore

In the concept of system/vendor build split, usually vbmeta.img
won't be built in a system-only build and/or a vendor-only build.
Instead, vbmeta.img will be generated later when combining system
and vendor artifacts.

  - system-only artifacts: system.img, system_ext.img,
                           product.img and vbmeta_system.img
  - vendor-only artifacts: boot.img, vendor.img, odm.img and
                           vbmeta_vendor.img

PRODUCT_BUILD_VBMETA_IMAGE can be used to disable building vbmeta.img.
However, it also disables vbmeta_system.img and vbmeta_vendor.img
generation because both are only depended by vbmeta.img.

This change adds both vbmeta_[system|vendor].img into droidcore,
so they will be built even if PRODUCT_BUILD_VBMETA_IMAGE is set
to false, when we're building system-only artifacts or vendor-only
artifacts.

Bug: 161425613
Test: sets PRODUCT_BUILD_VBMETA_IMAGE := false then build, checks
      vbmeta_system.img is generated but vbmeta.img is not.
Change-Id: I39d9819da4704195b0e1ee58d13c848ae97d474a