Change Coldstart type to boolean

Summary: Updated the coldstart type to boolean, as number is not required, its a boolean value.

Differential Revision: D14260678

fbshipit-source-id: 236a38e04fd31cab4a561f5e0365bc43ad98adea

src/plugins/fresco/api.js

@@ -58,7 +58,7 @@ export type ImageEvent = {
   startTime: Timestamp,
   endTime: Timestamp,
   source: string,
-  coldStart: number,
+  coldStart: boolean,
   viewport?: ViewportData, // not set for prefetches
 };