Add @TestApi to registerSystemAction and unregisterSystemAction

@SystemApi requires @hide, so @TestApi is needed for these to
be visible to CTS testing

Test: methods visible to CTS test
Bug: 152052560
Change-Id: Icd7f82f3859a1deccd41d2279d8f9722a42df957

core/java/android/view/accessibility/AccessibilityManager.java

@@ -1288,6 +1288,7 @@ public final class AccessibilityManager {
      * @hide
      */
     @SystemApi
+    @TestApi
     @RequiresPermission(Manifest.permission.MANAGE_ACCESSIBILITY)
     public void registerSystemAction(@NonNull RemoteAction action, int actionId) {
         final IAccessibilityManager service;
@@ -1315,6 +1316,7 @@ public final class AccessibilityManager {
      * @hide
      */
     @SystemApi
+    @TestApi
     @RequiresPermission(Manifest.permission.MANAGE_ACCESSIBILITY)
     public void unregisterSystemAction(int actionId) {
         final IAccessibilityManager service;