Add tests for downgrading apex to system version

Bug: 138423035
Test: atest StagedInstallTest#testStagedInstallDowngradeApexToSystemVersion_DebugBuild
Test: atest RollbackManagerHostTest#testApexOnlySystemVersionStagedRollback
Change-Id: I75f706dcfec315f7feec4b74a0bd1c8ddebf478a
6 files changed