UnifedEmail - Fix NPE in saveNotificationMap

Fixes an occasional NPE in NotificationMap when dismissing
a notification and value does not exist.

Change-Id: I9930826db421974d5224068f78d9ef1f7c1a7b2c
Merge branch 'cm-11.0' of https://github.com/DarkCyanogenMod/android_packages_apps_UnifiedEmail into cm-11.0