Crash when scanning WiFi QR code

Fix two crashes when scanning a WiFi QR code.
The crashes are due to the asynchronous nature of the scan/decode
process, where the result is delivered after the activity has been
destroyed.

Bug: 223640678
Test: manual, Settings -> Network & internet -> Internet
      Press the 'Scan QR code' icon
      Start to scan a WiFi QR code
      Just before then scan/decode completes, press Back
Change-Id: Ic19ca08590ed76ca9be46eabe3e0b17419fb0b44