libsnapshot_fuzzer: use protobuf

Use protobuf because it already has all the fuzzing implemenetations.
Delete fuzz_utils.

Pros:
- Fuzzing protobuf is faster; it is easy to achieve 4K exec/s
- It is more guided; protobufs are fuzzed using mutators, and mutators
  should have better knowledge of the structure of the fuzz data
- No more hand-written parsing code of the fuzz data. That code in
  fuzz_utils.h is deleted.
- Corpus data can be reused even after adding new fields in the protobuf
- Corpus data is human-readable and easily manually written (it is
  the text format of the protobuf)

Cons:
- The "actions" are "declared" in protobuf definition and "defined" in
  C++, so there's more boilerplate to write. Adding a new "Action"
  requires changes in both.

Test: run libsnapshot_fuzzer
Bug: 154633114
Change-Id: Idc2a6b2c087e370e4cfef53142a244b9b275389e

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