Allow ACTIVITY_TYPE_RECENTS to have multiple root tasks

A guest user having a third party launcher cannot open Recents.

When the preinstalled launcher is used, the built-in recents
implementation will be used (QuickstepLauncher.java) and
which means there won't be any task with activityType "recents"
in the display stack, because Home and Recents are the same.

When a third party launcher is used, the fallback recents activity
will be launched (com.android.quickstep.RecentsActivity).

When this RecentsActivity is created, a new task with activity type
"recents" is created and this task remains in the display stack
after the current user is switched to a guest user.
When we try to launch a new RecentsActivity it will not be allowed
since one activity of type "recents" is allowed to exist at the same time.

Solve this by allowing multiple root task for recents activity type.

Test: Install a 3p launcher, use that as default with a guest user and open recents
Bug: 304930756
Co-Author: Isak Lilja <Isak.Lilja@sony.com>
Change-Id: Ibacc05f7dd3ca6e835d7fea1ada87a21faea739a