[M120 Migration]Fix IMContextEfl crash issue during app exit

[M120 Migration]Fix IMContextEfl crash issue during app exit

During App exit, the RWHVAuraCommonHelperEfl destruction before the
IMContextEfl, after RWHVAuraCommonHelperEfl destruction, the rwh_helper_
of IMContextEfl been a wild pointers, once IMContextEfl use the rwh_helper_
wild pointers, it will cause crash.

On RWHVAuraCommonHelperEfl destruction, set the rwh_helper_ of IMContextEfl
to null to avoid crash.

refer:
https://review.tizen.org/gerrit/#/c/295867

Change-Id: Ife433c18d98ff0d4b39f310fafed89d03ee410ab
Signed-off-by: fangfengrong <fr.fang@samsung.com>