Make DockedStackDivider trusted

This is the divider that contains the handle in split-screen. It's
rendered by the system, so it's trusted.

Bug: 174847794
Bug: 158002302
Test: Tap around divider in split-screen, verify no toast or log about
      untrusted touches.
Change-Id: I9d5b56192bacd26fa2faff3c9a3b2f3bdcf0634f