Remove DexManager.

DexManager used to manage secondary dex use, which is now managed by ART
Service, so there is nothing for it to manage anymore.

This CL inlines some of its code, moves some to DexOptHelper, and
renames the rest to InstallScenarioHelper.

Bug: 258223472
Test: atest FrameworksServicesTests FrameworksMockingServicesTests
Test: Presubmit
Flag: EXEMPT cleanup
Change-Id: Ief7e3044980c32095a3a7b727fe854d076b3b1a0