Updating strings for unknown sources settings

Updated strings and removed some obsolete strings that were no longer
used. Removed unusued instance variable in AppStateInstallAppsBridge

Test: Manual

Bug: 36454399
Change-Id: I858bb388f311fd012527500cf35255d191b4dfb7

src/com/android/settings/applications/AppStateInstallAppsBridge.java

@@ -43,12 +43,10 @@ public class AppStateInstallAppsBridge extends AppStateBaseBridge {
 
     private final IPackageManager mIpm;
     private final AppOpsManager mAppOpsManager;
-    private final Context mContext;
 
     public AppStateInstallAppsBridge(Context context, ApplicationsState appState,
             Callback callback) {
         super(appState, callback);
-        mContext = context;
         mIpm = AppGlobals.getPackageManager();
         mAppOpsManager = (AppOpsManager) context.getSystemService(Context.APP_OPS_SERVICE);
     }