Floss: Use shell-words splitter for btclient

When splitting arguments, do the "shell" way using `shell-words` crate
that splits arguments and also tokenizes strings within quotes. This
will make parsing complex string arguments more easily, for example:

btclient> le-scan start-scan "complex filter within quotes"

Bug: 254870159
Tag: #floss
Test: Build btclient on Linux

Change-Id: I04568f8e497339ab6e96a7d38811b5b2bce764e0