Expire child sessions (3/n)

The child sessions need to be removed along with the parent to avoid
orphans.

* Add a new method to handle session expiry.
* Note we can't run expiration until all sessions are loaded
  and parent-child mapping is restored.

Bug: 209551994
Test: atest CtsRootPackageInstallerHostTestCases
Change-Id: Ib4299195b44b438ee98c4ce3d0eb23e1efda673d