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