Allow interact_across_users in app widget service

This change updates security policies in the app widget service to allow
packages that have interact_across_users permission to manage widgets on
behalf of other users.

Bug: 357621815
Test: verified able to host widgets from system user on hsum
Flag: android.appwidget.flags.security_policy_interact_across_users
Change-Id: I26b7274090096e54d6b6dea0a072e69bc914c609