Install system app in greatest priority partition

On Pixel 2 devices, /product is a symlink to /system/product. The
product partition has a higher partition precedence than the system
partition so the app should be installed as a system app on the product
partition.

This change also unifies methods for checking whether a file is within
a partition so we will paths will always be canonicalized before the
check.

Bug: 152522330
Test: update system app in system/product/privapp, uninstall updates,
      verify that the app was scanned as privileged
Change-Id: I646a5f293b977a78daa2102b73f1d3122f774a2a