floss: Modify set_pin and set_passkey interface

The `len` parameter is not useful in the Rust/D-Bus interface since an
the length is already provided in an array data type.

Bug: 202334519
Tag: #floss
Test: Build floss on Linux

Change-Id: I3acd58b996f0f158913e1f5f0ce20b652b6b26d3