Add default frame rate setting

Add a new priority PRIORITY_DEVICE_PEAK_REFRESH_RATE, and a new config
setting config_defaultRefreshRate which maps to that priority. This
allows an OEM to easily specify a default frame rate different from the
peak frame rate.

Bug: 148978450
Bug: 154648391

Test: - Added new unit tests to verify DisplayModeDirector handles min,
        peak, and default refresh rate settings correctly.

- Modified a Pixel 4 to set config_defaultRefreshRate to 60Hz, confirmed
  we default to 60, but that an app calling setFrameRate(90) switches us
  to 90.

- Confirmed that the "smooth display" and "force 90Hz" options on Pixel
  4 work correctly.

Change-Id: I00c82eea39a914b5b38506dd3143feb936749254