pdl: Change &String to &str for a simpler API

Test: atest pdl_tests pdl_rust_generator_tests_{le,be}
(cherry picked from https://android-review.googlesource.com/q/commit:4d31842ffc4bd4b67cbd89ae1b89ad2547f79980)
Merged-In: Icea91f71f7d22aa64ad191f084503ccbe9ac36a3
Change-Id: Icea91f71f7d22aa64ad191f084503ccbe9ac36a3