Enhanced import/export support.

1) Allow imports from the stock SMS database at any time.

2) Provide plaintext export support, in a format compatible with
   the "SMS Backup And Restore" app.

3) Fix the DB weirdness on encrypted restore that previously
   required killing the app.

src/org/thoughtcrime/securesms/DatabaseMigrationActivity.java

@@ -65,6 +65,11 @@ public class DatabaseMigrationActivity extends PassphraseRequiredSherlockActivit
     shutdownServiceBinding();
   }
 
+  @Override
+  public void onBackPressed() {
+
+  }
+
   private void initializeServiceBinding() {
     Intent intent = new Intent(this, ApplicationMigrationService.class);
     bindService(intent, serviceConnection, Context.BIND_AUTO_CREATE);