Add test for verifying if setVpnDefaultForUids() is called correctly

This commit verifies that
- If VPN network preference is set when
    - Ikev2VpnRunner is created
    - Retrying VPN
    - Updating exclusion list
- If VPN network preference is cleared when
    - Exiting VPN runner
    - NOT_RECOVERABLE happens

Bug: 231749077
Test: atest FrameworksNetTests:VpnTest
Change-Id: I71f48f18fd4556e04c131d5939afaa1ef5a0f814