Hold dex caches live in class table

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 f284d448e3edd428b6ade473d0993028638b2064)

Change-Id: Ie1b0b0cf835a998e19227cbb90014011a6cd40c4