Fix divider visibility issue on transient taskbar

On legacy case, we hide divider bar when gesture start, but this
might cause divider hidden when transient taskbar case.
We should follow the timing it actually start entering overview
then hide divider.

Fix: 261376202
Test: manual
Test: pass existing tests
Change-Id: Iae1339ae0b42033075dd840fc071ba28a7d3e13a