Write all amounts with the same format when exporting to QIF

We were writing some amounts with locale-less format and others with
user's locale. That make GnuCash desktop fail to import.

Now we always use the locale-less format. GnuCash always accepts them,
independently of the user's locale.

Fixes #655

Author: rivaldi8

app/src/main/java/org/gnucash/android/export/qif/QifExporter.java

@@ -38,8 +38,8 @@
 import java.io.OutputStreamWriter;
 import java.math.BigDecimal;
 import java.util.ArrayList;
-import java.util.Currency;
 import java.util.List;
+import java.util.Locale;
 
 import static org.gnucash.android.db.DatabaseSchema.AccountEntry;
 import static org.gnucash.android.db.DatabaseSchema.SplitEntry;
@@ -210,9 +210,10 @@ public List<String> generateExport() throws ExporterException {
                     if (quantity_denom != 0) {
                         quantity = quantity_num / quantity_denom;
                     }
+                    final Locale noLocale = null;
                     writer.append(QifHelper.SPLIT_AMOUNT_PREFIX)
                             .append(splitType.equals("DEBIT") ? "-" : "")
-                            .append(String.format("%." + precision + "f", quantity))
+                            .append(String.format(noLocale, "%." + precision + "f", quantity))
                             .append(newLine);
                 }
                 if (!currentTransactionUID.equals("")) {