Always create missing user contexts in WM Shell

Instead of assuming only the current user or its profiles can run tasks,
always create the needed user context. Also, as it seems the default
user (0) is often used, make sure we have its profile cached always.

Fix: 393054733
Test: atest UserProfileContextsTest
Test: atest WindowDecorTaskResourceLoaderTest
Test: atest DesktopTasksControllerTest
Flag: EXEMPT (bug fix)
Change-Id: I77ef353457924d74de713c0bcb9a98ef4cd7128a