Load engine when the first client is connected

Load engine when the first client is connected

- Issue:
Engine loads model when the engine is initializing, so engine always has
to load model, even if engine application does not need to load model in
some case.

- Solution:
To solve this problem, first, we need to change the timing for loading
engine. This is because engine has no information about clients when
engine is initialzing. Without any clients information, engine can not
decide whether the engine needs to load model or not.
This patch changes the timing for loading engine to when the first client
information is received. Through this patch, engine already has client
information when engine has to decide model loading.

Change-Id: Idfb67c1209dd17da2a1298ff5b82e4fec2cde18e
Signed-off-by: Suyeon Hwang <stom.hwang@samsung.com>