You can, and everyone always does, litigate invalidity before ITC. (I am an IP litigator who has done it.) Note that the ITC cannot itself invalidate patents like district courts do, but they can--and often do--decline to issue an exclusion order because they believe the patent at issue is invalid.
For what it's worth, this isn't limited to invalidity. For example, ITC findings that a product infringes also are not binding on courts. The reason for all this is that the ITC is an administrative agency rather than an Article III tribunal.
