Use USER_SYSTEM instead of USER_ALL when enabling rollback.

The rollback manager needs to know the user that the install session is
associated with. When installing a package for USER_ALL, the install
session is associated with USER_SYSTEM.

This fixes an IllegalArgumentException that would otherwise be thrown if
we ask for all the sessions associated with USER_ALL.

Bug: 130530159
Test: adb install RollbackTestAppAv1.apk
      adb install -i com.android.shell --enable-rollback RollbackTestAppAv2.apk,
          run from both primary and secondary users.

Change-Id: I4a778ffb96cd79875a54da9de9d348d4744a7efb