Some code cleanup (3/n)

Now sealLocked(), verifyNonStaged() and installNonStaged()
can retrieve child sessions on their own without being given
by the caller.

This improves cohesion of the methods.

Bug: 162386287
Test: atest StagedInstallTest
Change-Id: I33c6e9b68383b5348e839cfcfc72afd325452705