Restart verification for committed sessions that are not READY (2/n)

If reboot happens before a staged session reaches READY, we need to
rerun both APK verificatioin and pre-reboot verification.

* Add a member mBootCompleted to ensure we run verification after boot
  completed since we can't send broadcasts during booting.
* Rewrite startPreRebootVerification() using mBootCompleted to simplify
  the code.
* Fix StagingManagerTest since verifySession() is now the entry point
  for starting verification.

Bug: 181092466
Test: atest StagedInstallTest StagingManagerTest
Change-Id: I7a2708dc6bbc1e758d7e3fa87264552f36e1f42d