StagingManager: Support abandoning staged sessions

Add StagingManager and PackageInstallerSession functionality
for aborting an active staged session after being committed
but before reboot.

Also add abortActiveSession method to ApexManager.

Change-Id: Iba9ea29a2270df627b67fe27914793a442ee1a3d
Test: atest apex_e2e_tests
Bug: 124036308