DisplayModeDirector: remove debug_render_frame_rate_is_physical_refresh_rate

This sysprop is no longer needed as the changes in SF to respect
render frame rate are in place. In addition, we add a
`supportsFrameRateOverride` flag to DisplayModeDirector to match
the configuration in SF.

Test: atest DisplayModeDirectorTest
Test: manual (toggle Smooth Display from settings and observe refresh rate)
Bug: 241447632
Change-Id: Ia9a8571c2686ed6074344f6ec8609e169498d3df