Properly fix PerfettoTest.DetachAndReattach

The root cause of the flakiness was NOT due to session
timing out too early, as wrongly speculated by aosp/856951.
The root cause was the fact that the whole
attach->detach->disable sequence could happen too quickly
before the producer thread had a chance to write and flush
its data.
I managed to reproduce this by adding a usleep in
FakeProducer::StartDataSource()

Bug: 121352331
Change-Id: Ie192cc26ce408719ae0c029a9bef7dfe3103febe