Move and rename some members (1/n)

This is a preliminary change to apply READY/FAILED/APPLIED to
non-staged sessions.

* Move mSession* fields to PackageInstallerSession as they
  will be useful to both staged and non-staged sessions.
* Rename some members as they will be applicable to both staged
  and non-staged sessions.

Bug: 210359798
Test: atest StagingManagerTest \
            PackageInstallerSessionTest \
            CtsStagedInstallHostTestCases

Change-Id: Ie1b09aa4c7cbc843712c731a1b903d26684b2e30