pdl: Let UPDATE_SNAPSHOTS create new snapshots

Before, we had to create the files one-by-one, now we can regenerate
all of them automatically.

Test: atest pdl_tests pdl_rust_generator_tests_{le,be}
(cherry picked from https://android-review.googlesource.com/q/commit:7b828af494080e3680f8df7e00db017c2b592a4c)
Merged-In: Ia6f3947281d9f6d2c286d7dce68a75a2fa4ceebf
Change-Id: Ia6f3947281d9f6d2c286d7dce68a75a2fa4ceebf