Align rounded corners beside divider bar with display and task bar

Use the same way to draw rounded corners beside divider bar with
display and task bar to make sure all rounded corners share the same
style.

Bug: 196272915
Test: atest WMShellUnitTests
Test: manul check the rounded corners rendered in proper position.
Change-Id: I566ef5233dc345e0229fd9637c3b05705ba35864