pdl: Skip unnecessary computations in get_size

We might end up with adjustments again later as we parse packets with
more complex fields, but for now, this seems like a nice
simplification.

Test: atest pdl_tests
(cherry picked from https://android-review.googlesource.com/q/commit:fc84234d6d3483a7cbb8a17b7d1722014ef632f7)
Merged-In: I67e37c2de9c45db510c8e4013cee0c51dcf1f4bb
Change-Id: I67e37c2de9c45db510c8e4013cee0c51dcf1f4bb