Fix "invalid signal received" error

Fix "invalid signal received" error

- If the same API is called in several threads simultaneously,
  "api_activating[api]" value is reset when first response is come.
  As a result, second response will get "invalid signal received" error.
- This patch changes type of "api_activating" member variable from "gboolean" to "gint",
  and use it like reference count.

[Version] 0.3.41
[Issue Type] Bug fix

Change-Id: I08aa674d1d99d3a84af85b98e10dd047d944d3f9
Signed-off-by: Jeongmo Yang <jm80.yang@samsung.com>