Was calculating measured warmup time wrong

Bug: 6592923
Change-Id: I980c6fc69d78c569147ea44ce33a7f8a41739c91
diff --git a/services/audioflinger/FastMixer.cpp b/services/audioflinger/FastMixer.cpp
index 52effb2..b0af6ed 100644
--- a/services/audioflinger/FastMixer.cpp
+++ b/services/audioflinger/FastMixer.cpp
@@ -161,6 +161,7 @@
                 coldGen = current->mColdGen;
                 bounds = 0;
                 full = false;
+                oldTsValid = !clock_gettime(CLOCK_MONOTONIC, &oldTs);
             } else {
                 sleepNs = FAST_HOT_IDLE_NS;
             }