floss: Default run Floss if the config file is missing

On a brand new device the config file would be missing and BlueZ would
run by default, and then Chrome would switch the stack to Floss.
Since now Floss is fully launched, change the default to Floss.

Bug: 356808668
Tag: #floss
Test: mmm packages/modules/Bluetooth
Test: remove the config file and reboot, BlueZ doesn't run
Flag: EXEMPT, Floss-only change
Change-Id: Ic5a0a31896f8cabd58f655c50a9e38a9a2b0902a