Aviod divider surface memory allocated after boot

The divider surface buffer will be allocated even if the user does
not enter split mode. Avoid it by only copy surface when it show and
release it when it hidden.

Bug: 150190730
Test: reboot and check adb shell dumpsys SurfaceFlinger
Change-Id: Id70a2626b2a921cff6243d992c6b053e329ef92b