Persist sessions on the handler thread after sealing

So we won't block the binder thread by doing IO. It also
simplifies the code because this is always done at the beginning
of the install flow no matter it is a transferred session or not.

Bug: 162037825
Test: atest StagedInstallTest AtomicInstallTest
Change-Id: Ie041370320923d76d60748b19b1982306c053862