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] 0.4.112
[Issue Type] Bug fix

Change-Id: Ib98a2148e74e21e0b2d4d896b3b0f1baa4d0813b
Signed-off-by: Jeongmo Yang <jm80.yang@samsung.com>