Reland "Add a wrapper class PartitionBuildProp""

This reverts commit 6022545272.

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
Change-Id: I5c952cd2a976ba1a09ddc66d56c2b8b55a61986b
Test: unittests pass

tools/releasetools/check_target_files_vintf.py

@@ -80,8 +80,9 @@ def GetArgsForSkus(info_dict):
            '--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')