Made some inner classes static (For Efficiency)

(and made one explicitly private)

Closes #3932
// FREEBIE

src/org/thoughtcrime/securesms/database/IdentityDatabase.java

@@ -142,7 +142,7 @@ public class IdentityDatabase extends Database {
     }
   }
 
-  public class Identity {
+  public static class Identity {
     private final Recipients  recipients;
     private final IdentityKey identityKey;
 

src/org/thoughtcrime/securesms/database/TextSecureDirectory.java

@@ -201,7 +201,7 @@ public class TextSecureDirectory {
     }
   }
 
-  private class DatabaseHelper extends SQLiteOpenHelper {
+  private static class DatabaseHelper extends SQLiteOpenHelper {
 
     public DatabaseHelper(Context context, String name,
                           SQLiteDatabase.CursorFactory factory,

src/org/thoughtcrime/securesms/database/loaders/CountryListLoader.java

@@ -38,7 +38,7 @@ public class CountryListLoader extends AsyncTaskLoader<ArrayList<Map<String, Str
     return results;
   }
 
-  private class RegionComparator implements Comparator<Map<String, String>> {
+  private static class RegionComparator implements Comparator<Map<String, String>> {
     @Override
     public int compare(Map<String, String> lhs, Map<String, String> rhs) {
       return lhs.get("country_name").compareTo(rhs.get("country_name"));