Don't check allowBackup in PackageManager

During restore at install, PackageManager checks the allowBackup flag for
the package to determine whether or not to request a restore by calling
BackupManager::restoreAtInstall().

The above check isn't needed because B&R code verifies package elibility
before running backup / restore anyway. Furthermore, this check breaks
device-to-device migrations where we ignore allowBackup flag:
PackageManager doesn't know if the data is restored to the device from
cloud or directrly transferred from another device.

Remove the check altogether since it's not needed as described above.

Bug: 160407842
Test: 1. atest PackageManagerServiceTest
      2. Manual: Run restore at install for packages with
      allowBackup=true/false and verify the flag is respected.
Change-Id: I6ca61869129f5fabd40c01cd314cbf0f15e1f7fc