For static obtainBuffer(), do not set mUnreleased if acknowledging flush.

static audio tracks use obtainBuffer() to update position in start().

Bug: 22938515
Change-Id: I8ae32f6cce4d122386d2cf8982e158049b04ba9a
diff --git a/media/libmedia/AudioTrackShared.cpp b/media/libmedia/AudioTrackShared.cpp
index 6a51a76..caa84fb 100644
--- a/media/libmedia/AudioTrackShared.cpp
+++ b/media/libmedia/AudioTrackShared.cpp
@@ -932,7 +932,7 @@
     return (ssize_t) mState.mPosition;
 }
 
-status_t StaticAudioTrackServerProxy::obtainBuffer(Buffer* buffer, bool ackFlush __unused)
+status_t StaticAudioTrackServerProxy::obtainBuffer(Buffer* buffer, bool ackFlush)
 {
     if (mIsShutdown) {
         buffer->mFrameCount = 0;
@@ -970,7 +970,9 @@
     // it is always larger or equal to avail.
     LOG_ALWAYS_FATAL_IF(mFramesReady < (int64_t) avail);
     buffer->mNonContig = mFramesReady == INT64_MAX ? SIZE_MAX : clampToSize(mFramesReady - avail);
-    mUnreleased = avail;
+    if (!ackFlush) {
+        mUnreleased = avail;
+    }
     return NO_ERROR;
 }