Rewrote Japanese help pages

Translated the help pages from English to Japanese.

By the way, the app name in Japanese was defined as "QRコードスキャナー" ("QR Code Scanner" in Japanese) in res/values-ja/strings.xml.
The second commit changes it to "Barcode Scanner". The third commit changes it to "バーコードスキャナー" ("Barcode Scanner" in Japanese).

If "QR Code Scanner" is your intention, I will rewrite the help pages. If you prefer "Barcode Scanner" but not "バーコードスキャナー", ignore the third commit.

If you merge the commits, please also update the app name in Google Play.