Bluetooth: OBEX: Check if the added string is valid

There are types of string can be added by using OBEX adding header
function bt_obex_add_header_*(). One is byte sequence. Another is null
terminated Unicode text.

Add a function bt_obex_string_is_valid() to check if the added string
is valid. And add a function bt_obex_unicode_is_valid() dedicated to
check Unicode string.

Signed-off-by: Lyle Zhu <lyle.zhu@nxp.com>