system/stack/fuzzers: split client and server portions of fuzzers

We should fuzz only one of these at a time (and choose based on the
fuzzer-provided input) because:

1. it's unlikely a single input will meaningfully use both paths
2. a crash/bug on the first (server) path will prevent the second
   (client) path from triggering

We could split this into separate targets, but there's a lot of shared
setup and teardown logic.

Bug: 335257114
Test: m gatt-fuzzer, m sdp-fuzzer
Test: built and ran both fuzz targets
Flag: EXEMPT, fuzzing-only change
Change-Id: Ia1fd5b5d70bf6b58f1ab38d3da4a4c8673d1ec1b