Move graphics and JIT caches to DE storage.

We confirmed with the graphics and JIT teams that no sensitive
user data is written to these caches, so they're safe to point at DE
storage.

Since we don't have control over what is written by the app, we need
to keep the cache environment variable pointing at CE storage.

Fix ensurePrivateDirExists() to always return a path, instead of
returning null which can cause scary bugs.

Change packages.list to no longer canonicalize data paths, since
these fail when CE storage is still locked.

Bug: 27069522
Change-Id: Ifff64a036fa4aa1e61aa0dd98486bc711fbf8f4a