Rewrite #restoreSession (2/n)

Since we can retrieve child sessions from a parent session, we can
start restoration immediately when a parent session is received by
restoreSession().

Also remove #isMultiPackageSessionComplete which is no longer needed.

Bug: 167946370
Test: atest StagedInstallTest StagedInstallInternalTest
Change-Id: I8f253e0ff710e52a8e930ce1fe67e22d82a00b81