aaudio: initialize the callback buffer

This can prevent some loud noises during unit tests.

Also use std::unique_ptr

Bug: 154342174
Test: libaaudio/tests/test_various.cpp
Change-Id: I1c1dc0f4a6a29cf7d7a656392a81a325847de22d
diff --git a/media/libaaudio/src/client/AudioStreamInternalPlay.cpp b/media/libaaudio/src/client/AudioStreamInternalPlay.cpp
index 536009a..b50a512 100644
--- a/media/libaaudio/src/client/AudioStreamInternalPlay.cpp
+++ b/media/libaaudio/src/client/AudioStreamInternalPlay.cpp
@@ -268,11 +268,11 @@
     // result might be a frame count
     while (mCallbackEnabled.load() && isActive() && (result >= 0)) {
         // Call application using the AAudio callback interface.
-        callbackResult = maybeCallDataCallback(mCallbackBuffer, mCallbackFrames);
+        callbackResult = maybeCallDataCallback(mCallbackBuffer.get(), mCallbackFrames);
 
         if (callbackResult == AAUDIO_CALLBACK_RESULT_CONTINUE) {
             // Write audio data to stream. This is a BLOCKING WRITE!
-            result = write(mCallbackBuffer, mCallbackFrames, timeoutNanos);
+            result = write(mCallbackBuffer.get(), mCallbackFrames, timeoutNanos);
             if ((result != mCallbackFrames)) {
                 if (result >= 0) {
                     // Only wrote some of the frames requested. Must have timed out.