hwc: Add support for faking vsync

Can be set for debugging, valgrind, board bringup etc.
Also - we fail reading the sysfs vsync, we fall back and fake it.

Bug: 7301591
Change-Id: I9509fd7aa8862d5af7d521e29fe32bef9654168c
Signed-off-by: Iliyan Malchev <malchev@google.com>

Conflicts:

	libhwcomposer/hwc_vsync.cpp
1 file changed