r_submix : Fix minor issues in AIDL implementation

1. Update refinePosition for output stream
2. Add missing exitStandby on transfer
3. Add check for availableToRead in read functionality

Bug: 286914845
Test: atest VtsHalAudioCoreTargetTest
Change-Id: Ibab914e5e09474c2b55a6c64cc004ebc1bb6cb47
1 file changed