Fix improper unregistration of scanner for app that died

When scanner died, it should be unregistered as scanner, not as GATT
client. Otherwise app will not be able to start scan when it's
restarted.

This bug was introduced when separating GATT client from LE scan
clients.

Bug: 33556286
Test: start scanning, kill app, restart the app
Change-Id: If8da5512081e1986bbee7362984091f298500f4e