With content detection off, prefer default frame rate

When content detection is off, or when we have no layer information to
make a decision, prefer the default frame rate.

Bug: 154648391

Test: - Modified a Pixel 4 to turn off content detection, idle, touch,
        and power boost. Confirmed that, without this CL, we have bad
        behavior where calls to setFrameRate() persist after the layer
        goes away, and that this CL fixes that behavior.

- Modified and enhanced the "no layers" unit test in
  RefreshRateConfigsTest.cpp.

Change-Id: I2fd0d64a6ad369580cbb2ebf91cbbed7c31e0281