fingerprint: Adjust switch case logic

With the current switch case logic, it is difficult to update biod
system_api protos. system_api changes are upreved through regular rolls,
but then we have to fix the build failure caused by missing enum entries
in the uprev CL.

We should instead have the default case crash on receiving an unexpected
variant, instead of causing build to fail. The correct update sequence
should then be:
1. Update system_api proto in CrOS. Usage of new protos can be
   introduced, but they can't actually be used for communication
   between Chrome/CrOS yet as Chrome don't know how to handle them and
   will crash.
2. Update Chrome code to handle the new proto variants after system_api
   uprev.
3. After CrOS is shipped with new enough chrome version that includes
   2., the new proto variants can be used for communication between
   Chrome/CrOS.

Bug: b:321876084
Test: CQ
Change-Id: Icb053a20bc97459610caff0788eeecae6487248c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5223625
Commit-Queue: Howard Yang <hcyang@google.com>
Reviewed-by: Istvan Nagy <iscsi@google.com>
Reviewed-by: Renato Silva <rrsilva@google.com>
Cr-Commit-Position: refs/heads/main@{#1252490}