StackDivider: Make sure that changed stable insets get dispatched

Usually, content insets change whenever stable insets change,
but for the dock divider that is not the case because it only
ever receives stable insets.

Change-Id: Ic8e4d94d55773dbb0361156fce31b2c17a0dfebe
Fixes: 77911541
Test: rotate from landscape to seascape, make sure last stable insets for docked divider update.