audio: Report unknown stream positions explicitly

Instead of using 'STATUS_NO_INIT' in the case when stream
position can not be reported, use a special value for Position
fields. This streamlines processing of statuses on the client
side, by removing the need to treat 'STATUS_NO_INIT' specially.

Bug: 205884982
Test: atest VtsHalAudioCoreTargetTest
Change-Id: I13c9c8d165b632900ca76de144759ef7b9200eff