SF: Decouple VsyncModulator from Scheduler

Let VsyncModulator return phase offsets to SurfaceFlinger, and let the
latter propagate them to Scheduler.

Clean up identifiers and comments for consistency and regularity, and
fix uninitialized atomics.

Parametrize clock to avoid sleeping in tests, and minimize boilerplate
in test cases.

Bug: 160012986
Test: systrace with debug.sf.vsync_trace_detailed_info
Test: libsurfaceflinger_unittest
Change-Id: I05a4279592d38fdd933aad48118b2de0135cd0ea