[WRTjs] Block new thread creation while prepare app

[WRTjs] Block new thread creation while prepare app

While handles security_manager_prepare_app(), if new thread is
spawned by ThreadPoolService. Then smack error is happened from
security_manager_prepare_app(), which validates smack labeling of
threads because the spawned new thread has still System::Privileged.

PlatformThread::BlockThreading() will block CreateThread() until
security_manager_prepare_app() done.

This is alternative solution of self smack labeling, which is deleted
from m108.

Change-Id: Idfefe542904c7fbed89ee6ae0b98b6dede28a052
Signed-off-by: DongHyun Song <dh81.song@samsung.com>