Skip all of NetworkAgentTest in instant mode.

Recently, new test methods were added to NetworkAgentTest which
fail in instant mode because they are not marked @AppModeFull.

Because instant apps can't use NetworkAgent (it's a system API,
and it needs the NETWORK_FACTORY permission which is signature
only), just mark the whole test @AppModeFull so this cannot
happen again.

Bug: 283897713
Test: test is skipped in instant mode
Change-Id: Ib6ff07d9fdc329d3421a077532b8a434323053f8