pdl: Rework reading of scalar fields

This CL reverts the code reading scalar fields to match the logic in
the Python backend. This approach will make it simpler to update the
two backends going forward.

Test: atest pdl_tests pdl_rust_generator_tests_{le,be}
(cherry picked from https://android-review.googlesource.com/q/commit:40b64d7fbda9621b46ad72033af044a6a7d86b7a)
Merged-In: I49ddb3dae8eab1bb00cf730eda50a051d326ce2c
Change-Id: I49ddb3dae8eab1bb00cf730eda50a051d326ce2c