Add missing @Retention(SOURCE) to UnsupportedDrmException.Reason

This can fix the following warning caused by commit a7aa84a8:

Warning: com.google.android.exoplayer2.drm.UnsupportedDrmException.Reason:
The typedef annotation should have @Retention(RetentionPolicy.SOURCE)

 Author:    Tony Guo <tony.guo.peng@gmail.com>
 Date:      Mon Feb 28 10:56:52 2022 +0800

library/core/src/main/java/com/google/android/exoplayer2/drm/UnsupportedDrmException.java

@@ -23,6 +23,8 @@ import static java.lang.annotation.ElementType.TYPE_USE;
 
 import androidx.annotation.IntDef;
 import java.lang.annotation.Documented;
+import java.lang.annotation.Retention;
+import java.lang.annotation.RetentionPolicy;
 import java.lang.annotation.Target;
 
 /** Thrown when the requested DRM scheme is not supported. */
@@ -33,8 +35,9 @@ public final class UnsupportedDrmException extends Exception {
    * #REASON_INSTANTIATION_ERROR}.
    */
   // @Target list includes both 'default' targets and TYPE_USE, to ensure backwards compatibility
-  // with Kotlin usages from before TYPE_USE was added.  @Retention(RetentionPolicy.SOURCE)
+  // with Kotlin usages from before TYPE_USE was added.
   @Documented
+  @Retention(RetentionPolicy.SOURCE)
   @Target({FIELD, METHOD, PARAMETER, LOCAL_VARIABLE, TYPE_USE})
   @IntDef({REASON_UNSUPPORTED_SCHEME, REASON_INSTANTIATION_ERROR})
   public @interface Reason {}