Decouple downgrade and optimization processes.

Extract the logic to downgrade apps from the logic to
optimize apps. This will make easier to add more logic for downgrade
applications as required for b/112144519

Bug: 112144519
Test: This change is cover by the integration tests in
BackgroundDexOptServiceIntegrationTests/src/com/android/server/pm/BackgroundDexOptServiceIntegrationTests.java

Change-Id: Ia3ec6198bd8bac9871372ae87994ca0c596e36d7