Move session contains apex/apk logic inside session object

We will need these methods in PackageInstallerSession in following CLs.
Instead of duplicating, it makes more sense to move them into PIS since
they are more cohesive with PIS than StagingManager.

Bug: 163037460
Test: atest StagedInstallTest
Change-Id: I00b1c8f1a2889076a9d146798c3092913a3bdbe7