Extract verication code to a new class (2/n)

* A new class (PackageSessionVerifier) to run pre-reboot verification
  for staged-sessions.
* StagingManager.commitSession() is called after pre-reboot
  verification is done which will then notify the observers
  for staged apex.
* Dead code will be removed in next CLs.

Bug: 161121612
Test: atest StagingManagerTest
Test: atest CtsAtomicInstallTestCases
Test: atest CtsPackageInstallTestCases
Test: atest CtsStagedInstallHostTestCases
Change-Id: I9c80c34543af1d7f363b12af4a3720d3e6d4e7aa