blast: fix 30Hz wake up issue

When playing 30Hz video, SurfaceFlinger was waking up at a rate of
60Hz. This is because hasFrameUpdate returned true after the update
was already applied. mCurrentStateModified was being cleared after
the frame was sent to HWC instead of when the buffer was latched.

By moving "mCurrentStateModified = false" to latch, the hasFrameUpdate
returns false so SurfaceFlinger doesn't schedule the extra wake up.

Test: SurfaceFlinger_test
Bug: 128520233

Change-Id: I2f18c6d03b81affb1235d6b466d665e711b1db8c