Improve ASurfaceTransaction_setFrameTimeline docs.

Also use a new typedef AVsyncId shared in AChoreographer and
ASurfaceControl.

Test: atest ASurfaceControlTest; atest ChoreographerNativeTest
Bug: 214063411
Change-Id: If24f144404038064de2ba72cca44572aca507e44
diff --git a/include/android/choreographer.h b/include/android/choreographer.h
index 0389e57..889ecff 100644
--- a/include/android/choreographer.h
+++ b/include/android/choreographer.h
@@ -39,6 +39,12 @@
  */
 typedef struct AChoreographer AChoreographer;
 
+
+/**
+ * The identifier of a frame timeline.
+ */
+typedef int64_t AVsyncId;
+
 struct AChoreographerFrameCallbackData;
 /**
  * Opaque type that provides access to an AChoreographerFrameCallbackData object.
@@ -203,7 +209,7 @@
 /**
  * The vsync ID token used to map Choreographer data.
  */
-int64_t AChoreographerFrameCallbackData_getFrameTimelineVsyncId(
+AVsyncId AChoreographerFrameCallbackData_getFrameTimelineVsyncId(
         const AChoreographerFrameCallbackData* data, size_t index) __INTRODUCED_IN(33);
 
 /**