Clean up and simplify the logic in dexopt

Make some order in dexopt logic by extracting independent parts in their
own helpers.

It will make further updates of dexopt simpler and easier to reason
about (e.g. for secondary dex files).

Test: device boots
      adb shell cmd package compile -m speed|speed-profile packageName
      install new packages

Bug: 32871170

(cherry picked from commit 4a391f80)

Change-Id: If0c96174e904519a4332e54553690b691ff63149