alsa_sound: Changed mDevices to mCurRxDevice for StreamOut

- mDevices will be updated during route call and
  current device can change in between the start
  and write call of stream.
- In which case only the mCurRxDevice was updated
  and during write the device would not be updated
- This would create problem when switch device happens
  in session where it was alreasy set and device switch
  is ignored.
- here we use mCurRxDevice so even if there is a switch
  device in between the cur device will be used for
  routing in Stream

Change-Id: I94af59d7c8735b09e9adf2683e31533bf0f2be8e
CRs-Fixed: 490814