Core: Use ro.build.date to signal mIsUpgrade

M: We use a static fingerprint that is only changed when a new OEM build is released, so every flash shows Android is starting instead of upgrading.  This will fix that.

N: even though we dont have the dexopt sceen on N, this is still needed to delete the correct caches, and grant/deny specific runtime permissions like a true oem update would do

Updated for Nougat By: BeansTown106

Change-Id: I0e3ed5c8f0351e48944432ae6a0c5194ddeff1fa