Block unpriviledged apps which create keepalives with null fd

Currently, socketKeepalive implementation is accepting null fd
due to backward compatibility with legacy packet keepalive API.
However, due to lack of the fd, the service cannot guarantee the
port is not reused by another app if the caller release the port
for any reason.

Thus, grant the null fd access only for priviledged apps.

This commit also address some comments from aosp/918533.

Bug: 126699232
Test: atest FrameworksNetTests
Change-Id: I0baf582ff4ca8af6082c3754e8dfbcd867f39792