SF: use FenceTime when possible

Fence::getSignalTime is calling a system call everytime,
where FenceTime caches the signal time.

This shows reduction in simpleperf for the main thread
2.23% -> 1.25%
Test: simpleperf for PIP + Notification shade expansion
Bug: 184378996

Change-Id: I182db2ddfcb7fdbde758f5d87357a16e60c1bb07
diff --git a/services/surfaceflinger/Layer.h b/services/surfaceflinger/Layer.h
index 421a107..3f7a75f 100644
--- a/services/surfaceflinger/Layer.h
+++ b/services/surfaceflinger/Layer.h
@@ -261,6 +261,7 @@
         sp<GraphicBuffer> buffer;
         client_cache_t clientCacheId;
         sp<Fence> acquireFence;
+        std::shared_ptr<FenceTime> acquireFenceTime;
         HdrMetadata hdrMetadata;
         Region surfaceDamageRegion;
         int32_t api;