Now #mStagedSessions stores only parent sessions (4/n)

1. Don't store sessions until #commitSession or #restoreSession
   is called.
2. The stored sessions will be used by pre-reboot verification to check
   overlapping later.

Bug: 168268518
Test: atest StagedInstallTest
Change-Id: I945d2b92d9194f89493adf616231f8438b1c10b6
2 files changed