Turns on building vbmeta.img when merging if avb_enable=true.

Needed after commit 82182257
which skips regenerating vbmeta.img if avb_building_vbmeta_image!=true.
Partial builds that disable vbmeta generation do not set this field.

Bug: 169892879
Test: Run merge_target_files.py on builds that have avb_enable=true but
      PRODUCT_BUILD_VBMETA_IMAGE=false. Observe vbmeta.img generation.
Change-Id: Id498dbd28c909e19cd35178b95a0b5dc22a1ac70