Do APK verification before committing sessions to StagingManager (1/n)

This allows us to share the verification flow between both staged and
non-staged sessions.

* Do APK verification before committing sessions to StagingManager.
* remove handlePreRebootVerification_Apk() and its friends which
  are dead code now. This fixes TODOs and reduces coupling between
  StagingManager and PackageInstallerSession.

Bug: 181092466
Test: atest StagedInstallTest StagedInstallInternalTest

Change-Id: I4f9f16471b8e321e1ded5ba6dc9d1cfb982e6bba