Expose SdkExtension in the system SDK

Also tweak the javadoc a little bit.

Bug: 137191822
Bug: 143937447
Test: treehugger
Change-Id: Iae1b025685344c2ffe4ed1089850b447c82a6b83
Merged-In: Iae1b025685344c2ffe4ed1089850b447c82a6b83
diff --git a/Android.bp b/Android.bp
index e89aee0..2af7ca4 100644
--- a/Android.bp
+++ b/Android.bp
@@ -563,6 +563,7 @@
         "core/java/android/annotation/Nullable.java",
         "core/java/android/annotation/IntDef.java",
         "core/java/android/annotation/IntRange.java",
+        "core/java/android/annotation/SystemApi.java",
         "core/java/android/annotation/UnsupportedAppUsage.java",
         "core/java/com/android/internal/annotations/GuardedBy.java",
         "core/java/com/android/internal/annotations/VisibleForTesting.java",