SF: Encapsulate frame targeting

Introduce FrameTargeter to isolate a display's per-frame metrics around
past/upcoming deadline targets. The Scheduler updates the FrameTargeter
on frame begin/end, whereas ICompositor (concretely SurfaceFlinger) has
read-only access via the FrameTarget interface.

For now, only instantiate the pacesetter's FrameTargeter.

The only functional change is that `earliestPresentTime` now takes into
account the case of targeting two VSYNCs ahead.

Bug: 241285475
Bug: 241285191
Test: Perfetto
Test: dumpsys SurfaceFlinger --scheduler
Test: atest libscheduler_test:FrameTargeterTest
Change-Id: Idf9f43b37f3479c94a478d154eaa46f43e0c6c9d