Add config to control whether UMO shows on hub

We no longer want to show the UMO on the hub on mobile devices. This
config will be off by default and enabled only on tablet.

When the config is off, we do not listen to the media state which means
the UMO will never show in the hub.

Bug: 398075772
Fixes: 398075772
Test: atest CommunalOngoingContentStartableTest
Flag: NONE, using config variable. No behavior change on existing devices
Change-Id: I744d75ce05b32c600e1358513d4bc7b3f91fb7ff