added methods to @TestApi

Added ActivityManager.switchUser(UserHandle),
Context.startActivityAsUser, and UserHandle.of to
@TestApi

Test: None
Bug: 135282431

Change-Id: Iefbaa3c8f02649dd7d01be3093fa47104b077f58

core/java/android/app/ActivityManager.java

@@ -4064,6 +4064,7 @@ public class ActivityManager {
      * @hide
      */
     @SystemApi
+    @TestApi
     @RequiresPermission(android.Manifest.permission.MANAGE_USERS)
     public boolean switchUser(@NonNull UserHandle user) {
         if (user == null) {

core/java/android/content/Context.java

@@ -1827,6 +1827,7 @@ public abstract class Context {
      */
     @RequiresPermission(android.Manifest.permission.INTERACT_ACROSS_USERS)
     @SystemApi
+    @TestApi
     public void startActivityAsUser(@RequiresPermission @NonNull Intent intent,
             @NonNull UserHandle user) {
         throw new RuntimeException("Not implemented. Must override in a subclass.");