Unconditionally copy the system build.prop into target_files archive

releasetools preconditions requires build metadata defined in
SYSTEM/build.prop even when building a system.img-less product.
Copy the SYSTEM/build.prop into the build artifact archive whenever
possible to make add_img_to_target_files happy.

Bug: 212486689
Bug: 213028932
Test: Dist build a system.img-less product. Product properties are
  read from SYSTEM/build.prop and no build error occurred.
Change-Id: I2ce7f8b8ae981eaf06e2d8d2485b55b4e975caa9