Set min_sdk to 29 for NetworkStack tests

This allows installing the test package and running it on a release
build device even if the test is built from a branch with a
in-development or newer SDK.
The tests may still fail once run, but this approach allows excluding
specific tests if they use newer SDK features instead of blocking the
whole test suite at install time.

Bug: 145183152
Test: atest NetworkStackTests NetworkStackIntegrationTests
      NetworkStackTests still doesn't pass against qt-release but at
      least the tests are runnable.

Change-Id: Ie0ca963e2771ef317db9176cad8ce2b1b73567a7