SF: Use content detection flag to protect the content detection policy

Somewhere in the process of migration we forgot to include the
use_content_detection_for_refresh_rate sysprop flag. This CL guards
the content detection policy with this guard.

Test: Turn the flag off. No content detection. Turn the flag on.
      Observe content detection.
Test: On Cuttlefish.
Test: Added unit test in RefreshRateConfigs.
Test: atest libsurfaceflinger tests
Bug: 150003390
Bug: 150212108
Change-Id: Icff9ab9ffd3604049dfe36efd5d3939e1d77b091
diff --git a/services/surfaceflinger/Scheduler/RefreshRateConfigs.cpp b/services/surfaceflinger/Scheduler/RefreshRateConfigs.cpp
index b876ccd..1765d2d 100644
--- a/services/surfaceflinger/Scheduler/RefreshRateConfigs.cpp
+++ b/services/surfaceflinger/Scheduler/RefreshRateConfigs.cpp
@@ -107,6 +107,12 @@
         return *mAvailableRefreshRates.back();
     }
 
+    // If there are not layers, there is not content detection, so return the current
+    // refresh rate.
+    if (layers.empty()) {
+        return getCurrentRefreshRateByPolicyLocked();
+    }
+
     int noVoteLayers = 0;
     int minVoteLayers = 0;
     int maxVoteLayers = 0;
@@ -272,6 +278,10 @@
 
 const RefreshRate& RefreshRateConfigs::getCurrentRefreshRateByPolicy() const {
     std::lock_guard lock(mLock);
+    return getCurrentRefreshRateByPolicyLocked();
+}
+
+const RefreshRate& RefreshRateConfigs::getCurrentRefreshRateByPolicyLocked() const {
     if (std::find(mAvailableRefreshRates.begin(), mAvailableRefreshRates.end(),
                   mCurrentRefreshRate) != mAvailableRefreshRates.end()) {
         return *mCurrentRefreshRate;