cast Path to string type

os.path.isdir, os.path.exists cannot handle Path type until python 3.5
so cat Path to string type

Bug: 138639389
Test: development/vndk/tools/image-diff-tool/compare_images_and_print.sh "-t path1 path2 -s SYSTEM -i"
Change-Id: I147eac6f2548558737409a7a93ac54f0053a072e

vndk/tools/image-diff-tool/diff.py

@@ -69,7 +69,7 @@ def main(all_targets, search_paths, ignore_signing_key=False):
         if os.path.join(target, s).lower() + os.path.sep in p.lower():
           return True
       return False
-    paths = [path for path in Path(target).glob('**/*') if valid_path(path)]
+    paths = [str(path) for path in Path(target).glob('**/*') if valid_path(str(path))]
 
     def run(path):
       is_native_component = silent_call(["objdump", "-a", path])