floss: Add ascii_to_string

Add a function in topshim/btif to convert an ascii slice (&[u8]) into
a Rust String. This refactors the previous conversion for bt_bdname_t to
use String::from_utf8 instead of using str.

Bug: 197021037
Tag: #floss
Test: Rust unit tests
Change-Id: I84e989759a2d29747f4fbaf128a84cc83fca63b4