Handle downgrade install for rebootless apexes correctly (1/n)

1. ApexManager::checkDowngrade() doesn't handle the downgrade flag.
2. Downgrade install is already handled by PMS.
3. This change is required for rollback to work.

Bug: 195517333
Test: atest ApexManagerTest StagedInstallInternalTest
Change-Id: I86e49f26fc72df18291a8a65c43091d4be9b9e05