Add startActivityAsUser to SystemApi

Normal applications do not have permissions to use this method.

Test: m
Bug: 112869080
Merged-In: I693b3bf56f3be71f0790776e3aad5694717786ef
Change-Id: Ibe91c1bc4c94883a313e799cb0a37db7696fc62e

core/java/android/app/Activity.java

@@ -4735,7 +4735,7 @@ public class Activity extends ContextThemeWrapper
     /**
      * @hide Implement to provide correct calling token.
      */
-    @UnsupportedAppUsage
+    @Override
     public void startActivityAsUser(Intent intent, UserHandle user) {
         startActivityAsUser(intent, null, user);
     }

core/java/android/content/Context.java

@@ -1711,7 +1711,7 @@ public abstract class Context {
      * @hide
      */
     @RequiresPermission(android.Manifest.permission.INTERACT_ACROSS_USERS_FULL)
-    @UnsupportedAppUsage
+    @SystemApi
     public void startActivityAsUser(@RequiresPermission Intent intent, UserHandle user) {
         throw new RuntimeException("Not implemented. Must override in a subclass.");
     }

core/java/android/content/ContextWrapper.java

@@ -386,7 +386,6 @@ public class ContextWrapper extends Context {
 
     /** @hide */
     @Override
-    @UnsupportedAppUsage
     public void startActivityAsUser(Intent intent, UserHandle user) {
         mBase.startActivityAsUser(intent, user);
     }