GraphicsStatsValidationTest: fix a bug that the scheduleVsync is called at wrong time

c.a.s.cts.device.graphicsstats.DrawFramesActivity.updateColor() will
call scheduleVsyncLocked indirectly. This will cause the frame
to be scheduled on wrong Vsync.

This patch make the call to scheduleVsyncLocked happen at the correct time.

Bug: 146851008
Bug: 118714744
Test: atest com.android.server.cts.GraphicsStatsValidationTest
Change-Id: Id50d878ebab210bf6a2e8020b67e5d4e29aa789a