Some refactoring in BackgroundDexOptService.

Extract postOta/idle optimizations in their own method.
In preparation for adding the logic to handle secondary dex files.

Test: device boots, pacakges get compiled
Bug: 32871170

(cherry picked from commit be6a71a0)

Change-Id: Ie6cdd8461e7214f5de68bc9172f4171ebf72aa39