rusty-gd: Use spawn instead of block_on

Using block_on causes panic since the runtime is being used to drive
asynchornous tasks.

tokio::runtime::Handle has a spawn_blocking method but that can only be
called from async context. Use spawn instead.

With this change, I tested pairing with Pixel Buds and it works.

Bug: 171749953
Tag: #gd-refactor
Test: gd/cert/run --rhost DirectHciTest

Change-Id: Ife8904181bd570336e80b7980435808deba7f423