Don't build retrofit OTA package if vendor.img is unavailable.

We used to build retrofit full OTA package as long as
PRODUCT_RETROFIT_DYNAMIC_PARTITIONS is true. This doesn't work with AOSP
targets that have the flag set but without any available vendor image.

This CL detects such a condition and uses a separate flag to guard the
retrofit OTA building as well as the split super images generation.

Bug: 120852744
Test: `m -j dist` on blueline (w/ vendor images) and aosp_blueline (w/o
      vendor images).
Change-Id: I65726f24f8fc546be6802941a6a06590a3804c16