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

tools/releasetools/build_image.py

@@ -509,9 +509,9 @@ def ImagePropFromGlobalDict(glob_dict, mount_point):
   d = {}
 
   if "build.prop" in glob_dict:
-    bp = glob_dict["build.prop"]
-    if "ro.build.date.utc" in bp:
-      d["timestamp"] = bp["ro.build.date.utc"]
+    timestamp = glob_dict["build.prop"].GetProp("ro.build.date.utc")
+    if timestamp:
+      d["timestamp"] = timestamp
 
   def copy_prop(src_p, dest_p):
     """Copy a property from the global dictionary.