Make Display.Mode.isSynthetic TestApi

To use in CTS (FrameRateOverrideTest) for vrr device with synthetic
modes.

Bug: 353142444
Flag: TEST_ONLY
Test: atest FrameRateOverrideTest
Change-Id: Ic28956ac65f9642b9a78a31fff0111e28c6a69e0
diff --git a/core/api/test-current.txt b/core/api/test-current.txt
index 177b859..22bb450 100644
--- a/core/api/test-current.txt
+++ b/core/api/test-current.txt
@@ -3676,6 +3676,7 @@
 
   public static final class Display.Mode implements android.os.Parcelable {
     ctor public Display.Mode(int, int, float);
+    method public boolean isSynthetic();
     method public boolean matches(int, int, float);
   }
 
diff --git a/core/java/android/view/Display.java b/core/java/android/view/Display.java
index 15b0c13..1f7ed8b 100644
--- a/core/java/android/view/Display.java
+++ b/core/java/android/view/Display.java
@@ -2344,6 +2344,8 @@
          * SurfaceControl.DisplayMode
          * @hide
          */
+        @SuppressWarnings("UnflaggedApi") // For testing only
+        @TestApi
         public boolean isSynthetic() {
             return mIsSynthetic;
         }