Unregister network scan before deleting

Prevent potential race conditions when we receive the results of a
network scan after stopping all network scans. Unregister first to
ignore new scans and then stop scans

Test: atest FrameworksTelephonyTests
Test: try manual network scan and basic tests
Bug: 277876263
Change-Id: If1accf4303f9938497c73aea33864642ffae2fb9