A bit more refactoring in BackgroundDexOptService

Extract convenient methods to help with:
- adding a shell command to run the background dexopt job
- extending idleOptimizations to cover secondary dex files

Test: device boots, background job run.
Bug: 32871170

(cherry picked from commit c660475a)

Change-Id: I146e4faeea90f2d58144a5dba26ae7f90a36b402

Merged-In: I18fde62a1cb05c030a620722d5f217589beaefb1