Split abandon() (3/n)

Since the flow of abandoning a staged session is quite different
from that of a non-staged session, splitting the method into 2 parts
increase readability.

In next CL, we will change abandonStaged() to fix a bug where an
abandoned staged session is not correctly cleaned up.

Bug: 163976510
Test: atest AtomicInstallTest StagedInstallTest
Change-Id: Iab1cd2bc25865224383a21e793cf58336bca18a8