Add API to clear device suggestions and rename suggestion methods

Since APIs should generally return non-null values, have explicit
methods to clear device suggestions and also be notified when
suggestions are cleared.

Also, updated the names of the callback methods to make clear there are
multiple suggestions. Also updated the callback interface to reduce
ambiguity.

Flag: com.android.media.flags.enable_suggested_device_api
Bug: 414410102
Bug: 416016201
Bug: 392942321
Test: atest
Change-Id: Ibef0181d10a21d87dde152c8df6b3b9d6f4f1618