[Provider Model] Avoid running changes to WiFi level

- Because getWifiDrawable() has avoided the WIFI_LEVEL_UNREACHABLE(-1)
level design.

- But it might have a chance to changes in the short time after the
level checking above.

- Use the local variable to avoid the risk of changes in WiFi level
operation.

Bug: 210369886
Test: manual test
atest -c InternetAdapterTest \
         InternetDialogControllerTest \
         InternetDialogTest

Change-Id: Id3f27ac7e4cc52fa3eea640eddd34d49863df8d2