Use getOrCreateRootHomeTask in ActivityStartController

Allow #getOrCreateRootHomeTask() to take argument onTop for the position
to create the home stack at.

Also don't allow untrusted secondary display to have system decorations.

Bug: 153624902
Test: atest CtsWindowManagerDeviceTestCases:MultiDisplaySystemDecorationTests
Change-Id: I12d0cfcf2436da3619abd314febddb05610e57f2