Fix an issue in TrackRecord.

TrackRecord<T> implements List<T> but also has an add(T)
method which conflicts with List<T>#add as it has a different
return type. As nobody is using the return value of this now,
use the signature of List<T>#add to remove the problem.

Bug: 153613718
Test: NetworkStackTests FrameworksNetTests
Change-Id: Ie9eb4d7158e9583d4052e86f87062e3032b023e3