Send ACK for A2DP_CTRL_CMD_SUSPEND even if audio was no streaming

The A2DP_CTRL_CMD_SUSPEND command from the Audio HAL could be received
even if audio wasn't streaming before. Therefore, we should always
respond with an ACK once the command processing has been completed.

Bug: 109712592
Test: Manual: (1) Play music to Headset; (2) Select "This device" as output;
      (3) Stop playing music; (4) Select Headset as output

Change-Id: I0f5d52d14b50882931b9d19de1a2f25204fce19e