SF: Polish Fps class

For consistent comparison (which will later be changed to check ULP
distance instead of the magnitude-dependent threshold), express the
inequality operators in terms of equality, and avoid hashing floats.

Add Hz literals and (namespaced) operators.

Bug: 129481165
Bug: 185535769
Test: libsurfaceflinger_unittest
Test: dumpsys SurfaceFlinger --vsync
Change-Id: I79be5d2dd031218c4054774d2645efb337211538