Fix the caching of the compressed section.

A previous change fixed a race condtion, but caused a big memory
leak of the compressed section data. In order to fix this, move
the caching of the compressed section at the point that we cache
the elf image data itself.

Bug: 27152097

(cherry picked from commit c5017fdb)

Change-Id: Ib9e4342acf591fdaaf05fd7fabb6bc0b02b8f3f1