Update prefix

Author: tejasd

firebase-perf/src/main/java/com/google/firebase/perf/session/PerfSession.java

@@ -30,7 +30,7 @@
 /** Details of a session including a unique Id and related information. */
 public class PerfSession implements Parcelable {
 
-  private static final String SESSION_ID_PREFIX = "FPRS";
+  private static final String SESSION_ID_PREFIX = "FPR";
   private final String sessionId;
   private final Timer creationTime;