Fix build break for vendor freeze

1. Fix build break when rebuilding vendor images in
sign_target_files_apks, because of missing SYSTEM/build.prop:
  File ".../add_img_to_target_files.py", line 999, in <module>
  File ".../add_img_to_target_files.py", line 993, in main
  File ".../add_img_to_target_files.py", line 854, in AddImagesToTargetFiles
  File ".../add_img_to_target_files.py", line 229, in AddVendor
  File ".../add_img_to_target_files.py", line 391, in CreateImage
  File "add_img_to_target_files/common.py", line 394, in __init__
    self._fingerprint = self.CalculateFingerprint()
  File "add_img_to_target_files/common.py", line 537, in CalculateFingerprint
    self.GetBuildProp("ro.build.version.release"),
  File "add_img_to_target_files/common.py", line 457, in GetBuildProp
    raise ExternalError("couldn't find %s in build.prop" % (prop,))
ExternalError: couldn't find ro.build.version.release in build.prop

2. Support more scenarios (non-AB, RADIO...)

Bug: 192422274
Change-Id: I792b0a2c2354ed9312730e322ce28df49b3c2f7f