Keep tracing session alive in fuzzer

Commit 7e6446c3("Refactored surfaceflinger_frametracer_fuzzer")
added some more randomness to the fuzzer, but, by wrapping the
tracingSession unique_ptr into a block, also caused the tracing session
to be destoyed immediately.

Keeping the tracing session alive is important to exercise interesting
code path inside the fuzzed code. If the tracing session is dead,
tracing is effectively disabled and the tracing code in durface flinger
will not be covered.

Bug: 307601836
Change-Id: Ia1c4a76f09cf20fca56add4cab11a8cdfd579842