floss: mgmt: Don't restart Floss if the index doesn't present

If the index doesn't present then the start must fail. This patch only
aims to decrease the crash amount, no user facing issue is fixed.

Bug: 362366319
Tag: #floss
Test: mmm packages/modules/Bluetooth
Test: modprobe -r btusb  # btadapterd is not restarted immediately
      modprobe btusb     # btadapterd is then restarted
Test: btclient -c 'adapter enable' && modprobe -r btusb
      # btadapterd is not restarted after timeout
      modprobe btusb     # btadapterd is then restarted
Flag: EXEMPT, Floss-only change
Change-Id: Ie77fd55431c257308698d3b9a82673984bda2b8a