Fix errors from validate_target_files

We should skip lines that don't contain '=', e.g. empty lines.
Also, pop a warning instead of an error if a prop is defined
multiple times with the same value

Bug: 177240467
Test: unittest, run validate_target_files
Change-Id: Ifc9eadb91e5dda7170a19d875016e5a47e8fc592
diff --git a/tools/releasetools/test_validate_target_files.py b/tools/releasetools/test_validate_target_files.py
index e9896ae..48b563d 100644
--- a/tools/releasetools/test_validate_target_files.py
+++ b/tools/releasetools/test_validate_target_files.py
@@ -357,9 +357,6 @@
         'google/coral/coral:10/RP1A.200325.001/6337676:user/dev-keys',
         'ro.product.odm.device=coral',
     ]
-    input_tmp = ValidateTargetFilesTest.make_build_prop({
-        'ODM/etc/build.prop': '\n'.join(build_prop),
-    })
+    input_tmp = ValidateTargetFilesTest.make_build_prop(build_prop)
 
-    self.assertRaises(ValueError, CheckBuildPropDuplicity,
-                        input_tmp)
+    self.assertRaises(ValueError, CheckBuildPropDuplicity, input_tmp)