Make sure that filters affect the Description

JUnitParamsRunner caches the Description on first request but if
that happens before applying a filter then the Description never
gets updated. This fixes the problem by discarding the cached
Description forcing it to be regenerated next time it is
requested.

Bug: 36074730
Test: run cts -a arm64-v8a -m CtsLibcoreTestCases -t libcore.java.net.InetAddressTest#test_isReachable_by_ICMP
Change-Id: Ibc35c013c388f81fd5b69c553b167e3d11af6841