Limit bluetooth broadcast password to single byte characters

The bluetooth broadcast password is supposed to be single byte
characters, so crash hanppened when input too many double-byte
characters when connecting to other broadcast.
Limit the password to single byte characters to fix this issue.

Bug: 297840479
Test: Manual
Change-Id: Ifab89ee9a07f20d1f30aba17bed694b5a5da8480