audio: compress offload enhancements

1) Parse and set encoder metadata (delay, padding)
2) Call compress_next_track to allow partial drain
3) Do not flush output on returning succesfully from partial drain

Change-Id: I0fa1a2c968a5590dff9b6c58bd52bb111dcf3e9b
Bug: 8174410
diff --git a/hal/audio_hw.h b/hal/audio_hw.h
index 03f276f..2c7fffd 100644
--- a/hal/audio_hw.h
+++ b/hal/audio_hw.h
@@ -121,6 +121,8 @@
 
     stream_callback_t offload_callback;
     void *offload_cookie;
+    struct compr_gapless_mdata gapless_mdata;
+    int send_new_metadata;
 
     struct audio_device *dev;
 };