Don't special case for non-apex staged sessions.

So that we can reuse the existing call into rollback manager when
rollback is enabled.

Bug: 124221060
Test: adb install --staged file.apk
Test: adb install file.apex
Test: adb install-multi-package --staged file.apk
Test: adb install-multi-package file.apex
Test: adb install-multi-package file.apk file.apex
Test: atest StagedRollbackTest (test currently under development).
Change-Id: I8e8e417db11d07dd7404318240ee16235061ab4d