Revert creation of home stack from WindowState#getRootTask().

This causes an issue navigating back home after a restart. The root
cause is still being investigated.

Verified manually and w/ fix of launcher tests.

Bug: 149753418
Test: atest NexusLauncherTests:com.android.quickstep.DigitalWellBeingToastTest
Change-Id: Ied638ccea8854c77d88a805ef79200d5bd46e383