component/bt: A2DP code original in example project moved to component/bt directory;

1. move btif and A2DP source code from project directory to bluetooth directory;
2. some updates of audio source code;

components/bt/bluedroid/api/include/esp_bt_stack_manager.h

@@ -0,0 +1,14 @@
+#ifndef __ESP_BT_STACK_MANAGER_H__
+#define __ESP_BT_STACK_MANAGER_H__
+
+#include "esp_err.h"
+
+esp_err_t esp_bt_init_stack(void);
+
+esp_err_t esp_bt_deinit_stack(void);
+
+esp_err_t esp_bt_enable_stack(void);
+
+esp_err_t esp_bt_disable_stack(void);
+
+#endif /* __ESP_BT_STACK_MANAGER_H__ */