[UI Thread] Fix duplicated initialization of ecore signal handler

[UI Thread] Fix duplicated initialization of ecore signal handler

To prevent duplicated initialization of the ecore signal handler, this patch add
a new flag to check whether the ecore signal is initialized or not.
Before creating a new child process, the ecore signal handler blocks signals to
stop inheriting signal handlers. After creating a new child process,
the ecore signal handler unblocks signals.

Change-Id: I5f8e2efffec98e9cd3e4387420ceb7126aa01d84
Signed-off-by: Hwankyu Jhun <h.jhun@samsung.com>