Don't use ApexInfo.packageName in StagingManager

* ApexInfo.packageName has a misleading name since it represents name
  of apex module (e.g. com.android.tzdata), not the packageName from
  AndroidManifest.xml of an apex (e.g. com.google.android.tzdata). Later
  should be used in PackageManager-world while former is apexd-internal
  name.
* Restructured code a little bit to make the pre-reboot verification
  flow easier follow.

Test: atest CtsStagedInstallHostTestCases
Test: adb shell pull /system/apex/com.android.resolv.apex /tmp/resolv.apex &&
      adb install /tmp/resolv.apex &&
      adb reboot &&
      <verify staged session was applied>
Bug: 132324953

Change-Id: Iedfe87f6e70a2e1d2792487ac7b2891693ceb72a