Class: Get rid of most of java.lang.DexCache.

There was a fair amount of unnecessary code duplication between ART
and libcore. Most Class / reflect APIs had move to native anyway, so
this change moves the rest of them over and lets us delete a lot of
untested and somewhat bit-rotten code.

Test: test-art-host, cts -m CtsLibcoreTestCases

Change-Id: I5eab1fe95466991f23e88a920fea3924a0d3e459