only start latency measurement on successful phrase triggers am: f74101e6

Original change: https://googleplex-android-review.googlesource.com/c/platform/frameworks/base/+/22192238



Change-Id: I9ec726b3a59e6eceb87728ca3564884f5a2aca7b
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>