Move ro.build.characteristics to /product props

This property is not device-generic, as it's used for things like
whether the product is phone/tablet/other and whether it has an
sdcard or not.

Bug: 118618261
Test: adb shell getprop ro.build.characteristics for aosp_blueline
Change-Id: I6bbf1d4c36d7f37f92c67bebde58d72aba9d0852