Use the FuzzImplVSyncDispatch for fuzzing

This class seems to have been written for fuzzing, but it wasn't used
anywhere. Icdb80253436b4d0034fc20fcae8583efb7c30292 introduced new
attempts to use the VSyncDispatch, resulting in a null pointer
dereference in the fuzzer.

Override update, which is an abstract method added since this class was.

Ignore warnings for unused functions. surfaceflinger_scheduler_fuzzer.h
contains a static method, Now(), which is used by other code that pulls
in this header, but not by this executable.

Fixes: 270785319
Test: surfaceflinger_layer_fuzzer
Change-Id: I4e67e3f6383f466fbb092e7c7e9a0f867732c1f5