pdl: Use correct semantic for padding field

Padding field completes an array field by padding the size
to a fixed value using 0x00 fill

Test: atest --host --no-bazel pdl_python_generator_tes
(cherry picked from https://android-review.googlesource.com/q/commit:8d582a2e033569034d80c230278f9ee8ba4a4cd5)
Merged-In: Iee0271c7ea1b9ba9b8073585450edf6916d53adf
Change-Id: Iee0271c7ea1b9ba9b8073585450edf6916d53adf