Iterate over parent sessions (1/n)

Use #sessionContains(filter) to check if there is overlapping.

This allows us to store only parent sessions in #mStagedSessions and move
the call to #createSession from PackageInstallerService into StagingManager
in the future.

Bug: 168268518
Test: atest StagedInstallTest
Change-Id: I1d8f71149cd9957f45b6bcaee3e75dd68c8a45d4