Rewrite how we abandon stages sessions (1/n)

1. if abandon() is called before or after pre-reboot verification, it
   is safe to clean up the session immediately.
2. otherwise, it wait until notifyEndPreRebootVerification() is called
   to do the clean-up.

This is the 1st step to move all calls to StagingManager outside of
the lock. The goal is make it safe for StagingManager to call into
PackageInstallerSession methods without incurring deadlock.

Bug: 161765186
Test: atest StagedInstallTest AtomicInstallTest StagedInstallInternalTest
Change-Id: Ic37b58e621ba8fd0ba61c78b9ac44871fe0cefcb