SF: use frame rate override even when the physical range is single

When the physical range is a single rate (e.g. 120Hz), render rates
where discarded if they could be run at a lower physical rate
(e.g. 30fps of 60Hz). This CL enables these frame rates to be able to
save power by running on a slower rate.

Bug: 296079213
Test: play 30fps video while the device policy has a
      single physical refresh rate
Test: atest libsurfaceflinger_unittest

Change-Id: I4d6d6ddffd6004022d02329148709204ff9d57db
Merged-In: I4d6d6ddffd6004022d02329148709204ff9d57db