Expose android.os.Build.VERSION.ACTIVE_CODENAMES to CTS

So that CTS can handle more than one active codename.

Bug: 130764058
Test: atest CtsOsTestCases:android.os.cts.BuildTest#testBuildConstants
Change-Id: I671e28308723ee9e79ded01f12330e4efdbbbe17

api/test-current.txt

@@ -923,6 +923,7 @@ package android.net.util {
 package android.os {
 
   public static class Build.VERSION {
+    field public static final String[] ACTIVE_CODENAMES;
     field public static final int FIRST_SDK_INT;
     field public static final int RESOURCES_SDK_INT;
   }

core/java/android/os/Build.java

@@ -287,6 +287,7 @@ public class Build {
         /**
          * @hide
          */
+        @TestApi
         @UnsupportedAppUsage
         public static final String[] ACTIVE_CODENAMES = "REL".equals(ALL_CODENAMES[0])
                 ? new String[0] : ALL_CODENAMES;