Add missing attribute to compatibility file.

untrusted_app_visible_hwservice was an attribute that was meant to
give partners time to add their HALs to AOSP.  It was removed from mr1
and so needs to be accounted for in the compatibility mapping.

Bug: 64321916
Test: Builds with treble policy tests.
Change-Id: I359a842083016f0cf6c9d7ffed2116feb9e159c6

prebuilts/api/26.0/26.0.cil

@@ -5,6 +5,7 @@
 (typeattribute hal_wifi_keystore)
 (typeattribute hal_wifi_keystore_client)
 (typeattribute hal_wifi_keystore_server)
+(typeattribute untrusted_app_visible_hwservice)
 
 ;; types removed from current policy
 (type asan_reboot_prop)