Skip exit of bg runnable app even if multitasking is not supported

Skip exit of bg runnable app even if multitasking is not supported

Fix background support app's termination issue when multitasking is
false.
Even though multitasking is false, the app is background runnable app,
it must run as working in BG.

From XWALK, WebApplication::Suspend() has a logic that it check first
background_support_enabled() value. If it is false, check if its
multitasking option.

Change-Id: I1c93f8a78961e52ecf7bff731021612ee9954974
Signed-off-by: DongHyun Song <dh81.song@samsung.com>