Add onDeviceSuggestionUiVisible API

This API will be called whenever SysUI displays a component for device
suggestions. The API to notify other routers that the UI is visible will
not be public. However, proxy routers can listen to the callback in
order to update their provided suggestions if they choose to do so.

Flag: com.android.media.flags.enable_suggested_device_api
Bug: 404359713
Test: n/a, API is currently for internal testing
Change-Id: I936aa35fd54624d61c0104fbaac60b63b6313c5f