Bump the required SDK level to U for isUidNetworkingBlocked

This change is a follow-up from aosp/2970991. The kernel bug
can trigger crashes on some T devices. Therefore, bumping the
required API level is necessary.
No CTS modification since the check doesn't exist in
pre-U CTS releases.

Test: atest CtsHostsideNetworkTests:com.android.cts.net.HostsideNetworkCallbackTests
Bug: 326143935
Change-Id: I63796f25315ee9f966dd5bf0afd2240b1ed456e1