[M108 Migration][WRTjs] Lazy initialization of ContentMainRunner for wrt-loader

[M108 Migration][WRTjs] Lazy initialization of ContentMainRunner for wrt-loader

In case cold booting case, WRT initialization takes much time on
the low-end product. (5s ~ 7s)
Even though wrt-loader is prepared on cold booting sequence as soon
as possible, the candidate launch can be failed because wrt-loader
is not yet ready.

To make short the initialization time, this patch defers the jobs
of WRTContentMain's initialization with ecore_idler, and return
loader's create callback early to prepare wrt-loader.

Reference:
https://review.tizen.org/gerrit/284208/

Change-Id: Icb1be128397415903d3de8b3346a862a98e7f1e2
Signed-off-by: DongHyun Song <dh81.song@samsung.com>