Stop creating extra apk session during post-reboot install phase

Since verification logic has been decoupled from install logic, we can
now create an entry point between the two and directly install the
staged session while skipping the verification process.

Previously we used to create an extra apk-only session with
INSTALL_DISABLE_VERIFICATION flag attached to it.

Bug: 163037460
Test: atest StagedInstallTest StagedInstallInternalTest
Change-Id: I1dcfd897a98c4f518a8947e43540bd58457791be