SurfaceFlinger: use vsyncPeriod from HWC

Composer 2.4 onVsync callback provides a vsyncPeriod. Use that period
in VsyncReactor to know when a vsync transition is done.

Test: adb shell /data/nativetest64/libsurfaceflinger_unittest/libsurfaceflinger_unittest
Bug: 140201379
Change-Id: Ia255e3b1d722fd1a3e571ec2aeceb1e8569d44d4