SF: Refactor Scheduler mock injection

Factor out TestableScheduler setup to TestableSurfaceFlinger,
and instantiate a VSyncModulator, since the std::optional may
be dereferenced in tests.

Bug: 123530318
Test: libsurfaceflinger_unittest
Change-Id: Ic7c95024e63d861c5c5c53e4973995a4fd83a7d8