[M120 Migration] Migrate platform language related patches.

[M120 Migration] Migrate platform language related patches.

1.Provide current platform language for navigator.language

Web App uses navigator.language to set language information.
But sometimes get wrong language value when user change
language in system menu.
So change language set by always listening VCONFKEY_LANGSET key.

2.Fix crash in PlatformLanguageChanged,
if destructor called before it, may cause crash.

Reference:
 - https://review.tizen.org/gerrit/293445/

Change-Id: I37d76fa4bfddc297493765d47025ca57739ed58b
Signed-off-by: jiangyuwei <yuwei.jiang@samsung.com>