floss: Fix build faliure: non-void function does not return a value

Also enabled Werror in build.py and fixed a warning in bt_service.

Bug: 290844854
Bug: 361684591
Tag: #floss
Test: mmm packages/modules/Bluetooth
Test: USE=floss_upstream emerge-nami floss
Flag: EXEMPT, Floss-only changes
Change-Id: If832a37a4bd466b0862ca0c4773919b80239fb2f