Fix an error of 'Object has been destroyed'

Fix an error of 'Object has been destroyed'

When app is terminate by SIGTERM or tizen.application...exit(),
There comes a TypeError 'Object has been destroyed'

suspend() will be called again on quit() for backgroun-runnable
apps, but since this.mainWindow can be already destroyed, below
code can throw exception with destroyed object 'this.mainWindow'
 - addonManager.emit('lcSuspend', this.mainWindow.id);

It is enough to call setEnable() for suspending (if window is alive)

Change-Id: Ie2e99cf74ca412902e4456901dc8350f038e3067
Signed-off-by: DongHyun Song <dh81.song@samsung.com>