Only register Launcher back-callback when ROLE_HOME is held.

When using a 3P Launcher, i.e. when the ROLE_HOME is not held by QuickstepLauncher, there should be no back callback registered with SystemUI.

Bug: 374882166
Flag: EXEMPT bugfix
Test: Manual, i.e. verified back-callback registration updates in SystemUI and also verified that back-to-home navigations in Nova Launcher are responsive.
Change-Id: I03187f80a4c3c893a6529f0a22613f6fdfe7b7b3