Fix null issues with suggestions codepaths

Update the system server implementation to never return a null map of
suggestions, to align with nullability design guidelines for APIs. Also
update InfoMediaManager to check if the previous suggestion is null
before checking route IDs.

Test: ateast
Bug: 393216590
Flag: com.android.media.flags.enable_suggested_device_api
Change-Id: Icf943387aea26a796a950f2e2882a90d1e415f95