VCN CTS: Wait for an active sub ID before running tests

In the early stage during phone reboot #getDefaultDataSubscriptionId
may not return a valid data subId and cause tests to fail. To
mitigate these failures and make the tests less flaky, this patch
enables tests to wait for the active subscription ID to become
available before proceeding.

This patch also ensures that airplane mode is disabled before testing.

Bug: 331658552
Test: atest CtsVcnTestCases
Flag: TEST_ONLY
Change-Id: I41fe297c14433f5f64c1f71395f042027b565390