Fix incorrect initial dex cache size.

We were previously setting it to be sizeof(DexCacheClass) instead
of sizeof(DexCache). This was causing some problems with
compaction when I relied on the object sizes being accurate to
visit objects in the bump pointer space.

Bug: 8981901
Change-Id: Iede04763aced041986b1b239368fc867143ad70d
Merge "Fix incorrect initial dex cache size." into kk44