Properly support multi-user in NotificationSettingsRepository

(And also in every place that injected SecureSettingsRepository / SystemSettingsRepository).

This also:
* adds some methods to UserAwareSecureSettingsRepository (via the base class) and their tests.
* removes the UserAwareSecureSettingsRepository interface/impl/fake split.
* adds a UserAwareSystemSettingsRepository (same base class, but interacting with SystemSettings instead of SecureSettings).

Bug: 356099784
Test: manual, as described in b/356099784
Flag: com.android.systemui.user_aware_settings_repositories
Change-Id: Ic8d759de9cc93e59ae279ecce0c818b1f3d200e3
diff --git a/packages/SystemUI/aconfig/systemui.aconfig b/packages/SystemUI/aconfig/systemui.aconfig
index 83b7566..13c22d0 100644
--- a/packages/SystemUI/aconfig/systemui.aconfig
+++ b/packages/SystemUI/aconfig/systemui.aconfig
@@ -1508,6 +1508,16 @@
 }
 
 flag {
+  namespace: "systemui"
+  name: "user_aware_settings_repositories"
+  description: "Provide user-aware versions of SecureSettingsRepository and SystemSettingsRepository in SystemUI modules (see doc linked from b/356099784)."
+  bug: "356099784"
+  metadata {
+    purpose: PURPOSE_BUGFIX
+  }
+}
+
+flag {
     name: "notify_password_text_view_user_activity_in_background"
     namespace: "systemui"
     description: "Decide whether to notify the user activity in password text view, to power manager in the background thread."