Extract staged session code to a separate class (1/n)

Now StagingManager manages a collection of StagedSessions instead of
PackageInstallerSessions which might expose info not directly related to
staged sessions and not useful to StagingManager.

In next CL, we will move fields/methods that are related to staged
sessions into PackageInstallerSession#StagedSession to improve the
cohesion.

Bug: 166694095
Test: atest StagedInstallTest
Test: atest StagingManagerTest
Test: atest PackageInstallerSessionTest

Change-Id: I4eeeea280d10fc8f49a3aa053fd907be5c2e3706