installd: Refactor RunDex2Oat to reduce variable life span

Files are grouped in closer paragraphs. The intention is to make it
easier to replace input file descriptor with paths later when needed.

Bug: 161470356
Test: atest run_dex2oat_test
Change-Id: Ifb7171b8ece604a39353fd656b5b5ce84f2f308c