Validate cellular network in testSetAvoidUnvalidated() for accurate testing

* Modified testSetAvoidUnvalidated() to ensure the cellular network is
  validated before proceeding with default network switching tests.
* This prevents inaccurate test results when the cellular network is not
  usable, as the connectivity service won't switch to it if the Wi-Fi
  network becomes invalid.

Bug: 371818866
Test: TH
Change-Id: I458903392605a44470d5f226a36cfa983ec71aea
1 file changed