Add to lock/unlock focus Id mutex

Add to lock/unlock focus Id mutex

There are cases that focus or focus watch callback is used internally
for session backward compatibility. Because there has not been any mutex for
focus id, sometimes an unintended error happens accessed from another thread.
Now, a mutex for the focus id is added and operated to secure the critical section.

[Version] 0.4.3
[Issue Type] Bug fix

Change-Id: Ib4c721f49e80417a6b92e19e1bf61c81148bb1d4
Signed-off-by: Sangchul Lee <sc11.lee@samsung.com>