Make gatt_attr use opportunistic connection

This patch creates new connection mode - opportunistic connection. When
such connection is made, no call to gatt_update_app_hold_link_status
will be made when it's started.

This means that connecting and disconnecting in this mode won't trigger
disconnect timer. When other, app creates regular connection and then
disconnects, the physical connection might get disconnected.

Opportunistic connection is used only for code setting CCC right now.

Bug: 30186455
Change-Id: Ia5be7682b6c3dcb4993060f818dee603aef3e19e