ASurfaceControl: support framebuffer devices

There are Android devices that use a hwc2onfbadapter that allows
them to ship framebuffer devices but still pass VTS and appear to
the system as if they have hwc2.x*.

Unfortunately, these devices do not support a present fence.
This patch updates the documentation to let developers know that
not all devices will return a present fence.

Test: ASurfaceControlTest
Bug: 129880031
Change-Id: I81dd21dc088b1d2d73d1bdb9c46216ef2e171063
diff --git a/include/android/surface_control.h b/include/android/surface_control.h
index ef2ad99..abb8368 100644
--- a/include/android/surface_control.h
+++ b/include/android/surface_control.h
@@ -130,7 +130,7 @@
 /**
  * Returns a sync fence that signals when the transaction has been presented.
  * The recipient of the callback takes ownership of the fence and is responsible for closing
- * it.
+ * it. If a device does not support present fences, a -1 will be returned.
  */
 int ASurfaceTransactionStats_getPresentFenceFd(ASurfaceTransactionStats* surface_transaction_stats)
                                                __INTRODUCED_IN(29);