Use tokio::spawn instead of choosing the runtime explicitly

tokio::spawn chooses the correct current runtime the caller is in the
context in, so there is no need to explicitly specific topstack runtime.
This makes the code less dependent on a particular runtime.

Bug: 188718349
Tag: #floss
Test: manual - Build floss on Linux

Change-Id: Ided764f4f737bd98ae85b6c06ebe1539e03b3b2c