Reset flow event cache before each perfetto event

This was just an omission. The omission led to a performance
regression (code was still logically correct) where we will
create a new Flow object on every trace event. This was
expensive and led to GC churn.

Change-Id: Idb027748f1bf3a44e1dfae7c666be4688b2e9338
Test: atest PerfettoTest
Test: atest TracePerfTest
Bug: 303199244 
Flag: android.os.perfetto_sdk_tracing_v2