Hold dex caches live in class table

Prevents temporary dex caches being unloaded for the same dex file.
Usually this is OK, but if someone resolved a string in that dex
cache, it could leave stale pointers in BSS. Also it can use extra
memory in linear alloc if we allocate dex cache arrays multiple
times.

Bug: 29083330

(cherry picked from commit f284d448)

Change-Id: Ie1b0b0cf835a998e19227cbb90014011a6cd40c4