Move version code checking out of StagingManager (3/n)

* Make verification work for APEX.
* Now we have version code checking for both APK and APEX in the same
  place (verifyReplacingVersionCode).
* This allows us to share more common flows between staged and
  non-staged sessions.

Note ag/13732406 will fix failed CTS for we move version code
checking into package verification.

Bug: 181092466
Test: atest StagedInstallTest StagedInstallInternalTest
Change-Id: Ia059b29139a5e1e0cda4a3ca9061ab3385a576ae