Replace hidden UserHandle.isApp() usage for mainline

ConnectivityService is going to become a mainline module which
is not able to access hidden APIs. NetworkPolicyManager will be
part of connectivity framework sources which should also not
access hidden APIs. Thus, replace usage of UserHandle.isApp()
by public Process.isApplicationUid() instead.

Bug: 170598012
Test: m
Change-Id: I6eecc618d574352b924cb5f33f364e2dca9d730f