SurfaceFlinger: Make LayerHistory virtual class

This is a base CL for refresh rate selection algorithm enhancements
by allowing a different implementation of LayerHistory which will be
controlled by a switch.

Test: watch 24fps video and observe refresh rate at 60Hz
Bug: 147516364
Change-Id: Ifd12f6683bc96cd5aba04a8960b9235729820b49
diff --git a/services/surfaceflinger/Scheduler/LayerHistory.cpp b/services/surfaceflinger/Scheduler/LayerHistory.cpp
index 146ec1b..cf79d9f 100644
--- a/services/surfaceflinger/Scheduler/LayerHistory.cpp
+++ b/services/surfaceflinger/Scheduler/LayerHistory.cpp
@@ -34,7 +34,7 @@
 #include "LayerInfo.h"
 #include "SchedulerUtils.h"
 
-namespace android::scheduler {
+namespace android::scheduler::impl {
 
 namespace {
 
@@ -157,4 +157,4 @@
     mActiveLayersEnd = 0;
 }
 
-} // namespace android::scheduler
+} // namespace android::scheduler::impl