releasetools: do not remove dynamic partitions in system-only builds

* Before this commit, the generated `dynamic_partitions_op_list` in
  FullOTA packages always tries to remove all partitions and recreate
  them upon flashing. This makes it impossible to have a system-only
  "FullOTA" because vendor partition(s) are always removed.

* This commit detects if a build is vendor-less and disables every
  dynamic partition operation except `resize`, in order to keep the
  original content around after the flash. The change should not affect
  non-dynamic-partition or builds with vendor image included.

Change-Id: I0cded7f3b2958f35103d73d19b7fb5f292f6c17f
Signed-off-by: Jesse Chan <jc@lineageos.org>