Add a wrapper class PartitionBuildProp

The build prop for a partition used to be a simple key:value
dictionary. But we need more fields to hold the alternative build
props overriden by the 'import' statement. Therefore, add a new
class as a wrapper for these props first.

Bug: 152167826
Test: unittests pass
Change-Id: I2fe7e93a2f4de8e55f5f8051b000b96b5efdc85a
diff --git a/tools/releasetools/check_target_files_vintf.py b/tools/releasetools/check_target_files_vintf.py
index b3d491f..95d09cc 100755
--- a/tools/releasetools/check_target_files_vintf.py
+++ b/tools/releasetools/check_target_files_vintf.py
@@ -80,8 +80,9 @@
            '--property', 'ro.boot.product.vendor.sku=' + vendor_sku]
           for odm_sku in odm_skus for vendor_sku in vendor_skus]
 
+
 def GetArgsForShippingApiLevel(info_dict):
-  shipping_api_level = info_dict['vendor.build.prop'].get(
+  shipping_api_level = info_dict['vendor.build.prop'].GetProp(
       'ro.product.first_api_level')
   if not shipping_api_level:
     logger.warning('Cannot determine ro.product.first_api_level')