Improve TopActivityFlag documentation in AppCompatTaskInfo

Better documentation for @TopActivityFlag.

Flag: EXEMPT Small fix
Bug: 376426159
Test: m

Change-Id: I7d0a6650d7e13b218cbe7a3653b7783d73b12b61
diff --git a/core/java/android/app/AppCompatTaskInfo.java b/core/java/android/app/AppCompatTaskInfo.java
index 6879458..009cd72 100644
--- a/core/java/android/app/AppCompatTaskInfo.java
+++ b/core/java/android/app/AppCompatTaskInfo.java
@@ -101,7 +101,6 @@
     @Retention(RetentionPolicy.SOURCE)
     @IntDef(flag = true, value = {
             FLAG_UNDEFINED,
-            FLAG_BASE,
             FLAG_LETTERBOX_EDU_ENABLED,
             FLAG_ELIGIBLE_FOR_LETTERBOX_EDU,
             FLAG_LETTERBOXED,
@@ -115,6 +114,10 @@
     })
     public @interface TopActivityFlag {}
 
+    /**
+     * A combination of {@link TopActivityFlag}s that have been enabled through
+     * {@link #setTopActivityFlag}.
+     */
     @TopActivityFlag
     private int mTopActivityFlags;