Cleanup how we handle abort of active apex session

- Removed throwing PackageManagerException from abortStagedSession since
it was masking actual failure reason in old code and in this CL, it's no
longer needed.
- Added justification for ignoring failed attempt at aborting active apex
sessions from apexd.
- Created ensureActiveApexSessionIsAborted for reuse. Will be used on
follow up CLs.
- Consolidated cleanup logic execuated on verification failure into
onPreReboootVerificationFailure. Previously, we did not clean up active
apex sessions in apexd in all scenarios.

Bug: 162294757
Test: atest ApexTestCases
Test: atest StagedInstallTest
Change-Id: I58d89be3a1712a51439e222cee4dbb1bec8c28f4