CtsVerifier: only write one loopback latency result

Recording multiple routes can cause the entire report file
to be rejected.

Copied from ag/23742693

Bug: 239039214
Test: Run AudioLoopback test.
Test: Save report log.
Test: Verify that only one new audio_loopback_latency_activity record
Test: was written.
Change-Id: Ic9856ff319da36792a09370685172b6686b14e2b
Merged-In: Ic9856ff319da36792a09370685172b6686b14e2b