Lock display refresh rate when primary range is a single rate.

When DMS enforces a fixed refresh rate to hide user-visible flickering,
we must respect that refresh rate to avoid unnecessary switches during
scenarios such as idle and touch boost.

We must still respect app votes when they explicitly provide a frame
rate.

Bug: 156315203
Test: Exoplayer
Test: Modified version of SetFrameRateTest to simulate 60hz video
Test: libsurfaceflinger_unittest
Change-Id: I5fe974ddaff30e2f02e381cb63e0a6418834fdb5