Abort apexd session only if staged session contains apex

* Always abort staged session. It's fine if aborting an apexd session
  fails. Apexd will wipe out metadata about previous sessions when
  committing a new one any way.
* Don't fail if there is no apexd session associated with this staged
  session. This can happen if abortCommittedSession was called twice.
  In this case second call will be a no-op. Which feels like a desired
  behaviour.

Test: adb install --staged foo.apk && adb shell pm install-abandon
Bug: 128108135
Fixes: 128108135
Change-Id: I016534d957ea559289a2e3d97e603a5e4d146e35