SF: add disabled VRR DispSync impl (VSyncReactor)

Adds a DispSync implementation called VSyncReactor that
is built on better tested components that should be
capable of evolving with the MRR and VRR plans in R and S.

Implementation is currently disabled, but can be enabled
by failsafe switch:

ENABLE:
setprop debug.sf.vsync_reactor 1
stop;start

DISABLE:
debug.sf.vsync_reactor not set, OR
setprop debug.sf.vsync_reactor 0
stop;start

while testing is being conducted (still flushing out bugs).

Fixes: 140303479
Test: check that DispSync1.0 is still active
Test: set property and check that DispSync2.0 turns on

Change-Id: I668e34e8880d0aa08222d1028bc874a2ab6a7339