Allow StagingManager to abort individual staged session

Now that apexd can stage multiple sessions, we need to be able to abort
specific staged session during pre-reboot verification. The only need to
abort activated session while resuming staged session during boot.

Bug: 141843321
Test: atest StagedInstallTest
Change-Id: I97d4216e1fe9ec6c98474ec12f4046b002d9edda
Merged-In: I97d4216e1fe9ec6c98474ec12f4046b002d9edda
(cherry picked from commit 44ad95be)