Only add divider when it need to show

By previous memory patch, the divider will always call update when
enter split when means it always add divider view when showing. So
we can reduce some update call to avoid any unnecessary surface
memory allocate.

Fix: 150190730
Test: Check split mode rotate normally and dump SF to check divider
      memory status
Test: Use forest to test cts/framework/gce-presubmit-wm-cloud-tf-3
Test: Use acloud to test cts/framework/gce-presubmit-wm-cloud-tf-3
Change-Id: I0aa4ad560eff6b64adc20cf2c04c3a61798c56ba