[art] Compile secondary dex files in isolation

The class loader context adds the overhead of extracting in-memory and
possibly extra verification which is being frowned upon when the loading
happens on the UI thread. Revert the correct fix of compiling secondary
dex files with their context until b/64530081 is done.

Partial revert of commit 659a7dcc.

Test: m test-art-host
Bug: 64530081
Bug: 66984396

(cherry picked from commit 2ea7ad1e)

Change-Id: I9b9c0a42e532fc057a36bc241127dd8a147b9abd