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
(cherry picked from https://android-review.googlesource.com/q/commit:95f9232923139f38d802054f5df27a73f1328a09)
Merged-In: Ibab914e5e09474c2b55a6c64cc004ebc1bb6cb47
Change-Id: Ibab914e5e09474c2b55a6c64cc004ebc1bb6cb47