secretkeeper: add fuzzer

Move nonsecure implementation code out of main.rs and into a new
secretkeeper_nonsecure library, to allow re-use in both the nonsecure
service and the new fuzzer.

The fuzzer drops the `LocalTa` instance on each input, so adjust the
error path to cope with this happening (the channels will fail).

Test: build, fuzz
Bug: 291228655
Change-Id: Ibe2ca5db9a9c8eed129ac5a90de85b933ab256a0