Hide 'Optimizing apps' dialog if not M->N upgrade

Recent changes reduced the amount of time spent optimizing apps at
post-OTA boot. Instead of briefly showing the 'Optimizing apps'
dialog, we hide it completely.

The dialog is still shown for OTAs from a pre-N version of Android
since they can take a non-negligible amount of time (still much less
than in M).

Bug: 27350503
Change-Id: I5f6dae0c69799e1fc522f36453d2671b333bef0e