Debounce multiple rapid suggestion requests

When UMO becomes visible, different listeners that check the visibility
state will all notify local media manager. We should throttle the
visibility updates so routes don't get erroneously notified.

Test: local test
Bug: 416745583
Flag: com.android.media.flags.enable_suggested_device_api
Change-Id: Iba689f45adf59f14d2b5ca5b39e5d6e07bf8120c