Unhide LinkProperties, NetworkCapabilities, NetworkRequest APIs

These methods are marked to @UnsupportedAppUsage APIs since
Android Q. But some system apps still need them to set/get
necessary network or request information. Hence, make them to be
public or system APIs.

Bug: 120448492
Test: atest FrameworksNetTests
Change-Id: I95a44daef5615e290b40d0796ca183b88ad8a63f