[Application] Fix cannot get application info from pkgmgr db.

Since xwalk already storage [tizen_app_id] and [package_id] of wgt
to pkgmgr db, so that we cannot get application info from pkgmgr db
by [app_id].

And xwalk will send [tizen_app_id] to extensions, so that we can
get [package_id] from [tizen_app_id] and get application info from
pkgmgr db according to the [package_id].

BUG=XWALK-2110
I'd suggest use hard code of `6` instead of compute `std::string("xwalk.").length()` each time.