API for whether Bubbles can show notifications

Introduces a new API method in Bubbles to indicate whether
it can handle bubble notifications.

This will be wired up in a follow up to the Notifications
API to determine when a HUN should be suppressed.

Flag: ACONFIG com.android.wm.shell.enable_bubble_bar DEVELOPMENT
Bug: 340532785
Test: Manual
Change-Id: I0ac6848995df2f17906e2b5817f17c4e6da87acc