Settings: Fix value from being overwritten with null.

After awhile this value becomes null for unknown reasons.
Switch back to a non-changing static final for the non-multi-sim case.

Change-Id: I9248a545ed35ec10048781b21a3e6a6b17e96d74
Merge branch 'kitkat' of https://github.com/AOSPA-legacy/android_packages_apps_Settings into HEAD