SF: Refactor event dispatch

This CL refactors the EventThread loop to simplify control flow, remove
redundant state, avoid gratuitous allocation, and improve readability.

It also adds a dumpsys flag for VSYNC information.

Bug: 74619554
Test: libsurfaceflinger_unittest
Test: dumpsys SurfaceFlinger --vsync
Change-Id: I2d875349031ae2869fb370a8a090c71b0e181482