Add task-id metadata constant

This can be used by alternate front-ends to associate
surfaces with tasks.

Bug: 122925737
Test: phone boots and surfaces still fling
Change-Id: If8e42908e61c4a99d1e53a006c04fe7c036c437d
diff --git a/libs/gui/LayerMetadata.cpp b/libs/gui/LayerMetadata.cpp
index c8a2b07..745433a 100644
--- a/libs/gui/LayerMetadata.cpp
+++ b/libs/gui/LayerMetadata.cpp
@@ -105,6 +105,8 @@
             return StringPrintf("ownerUID%s%d", separator, getInt32(key, 0));
         case METADATA_WINDOW_TYPE:
             return StringPrintf("windowType%s%d", separator, getInt32(key, 0));
+        case METADATA_TASK_ID:
+            return StringPrintf("taskId%s%d", separator, getInt32(key, 0));
         default:
             return StringPrintf("%d%s%dbytes", key, separator,
                                 static_cast<int>(mMap.at(key).size()));