Restore WATCH_APPOPS permission for systemui

This permission is necessary for Q as it allows systemui to tag
notifications of apps that are using certain AppOps. This is a P
feature that has regressed when the permission was made privileged.

This is used by AppOpsController in order to obtain updates on any apps
(onOpActiveChanged, onOpNoted). There is no other way to obtain the
updates from apps that are not systemui, as specified in AppOpsManager.

Talked to moltmann@ to verify that this is the correct API to use for this feature.
I'll remove this permission once SystemUI does not need it anymore.
In particular, if AppOpsController does not require it for being notified of AppOps changes.

Fixes: 134747188
Test: build
Test: dumpsys SystemUI. Check that AppOpsController is getting updates.
Change-Id: I08cca4361a7fbfa8b2eb419f1459b0b8bce93a5f