settings: fix value-len-get while base64 is enabled

While base64 encoding was enabled settings_val_get_len_cb() returned
encoded length instead of decoded value length.

This patches introduce procedure which calculates the value length
properly. For that additional read of the end of the encoded value is
required for checking how many form last 3 value bytes wear encoded.

fixes #12122

Signed-off-by: Andrzej Puzdrowski <andrzej.puzdrowski@nordicsemi.no>