floss: Fix the PID extract function

After https://r.android.com/3205038, the function always returns None
because the path doesn't contain any "/".

Bug: 356808668
Tag: #floss
Test: mmm packages/modules/Bluetooth
Test: Deploy to CrOS and reboot, BT can start
Flag: EXEMPT, Floss-only changes
Change-Id: I99208b1a8509336da71925fef7a05a159ed4d8ed