workloads/uxperf: change message format

Do not concatenate start/stop markers to the action string with a '_',
delimit them with a space instead. This will make easier to parse the
message afterwards.

wa/framework/uiauto/app/src/main/java/com/arm/wa/uiauto/ActionLogger.java

@@ -48,13 +48,13 @@ import android.util.Log;
 
         public void start() {
             if (enabled) {
-                Log.d("UX_PERF", testTag + "_start " + System.nanoTime());
+                Log.d("UX_PERF", testTag + " start " + System.nanoTime());
             }
         }
 
         public void stop() throws Exception {
             if (enabled) {
-                Log.d("UX_PERF", testTag + "_end " + System.nanoTime());
+                Log.d("UX_PERF", testTag + " end " + System.nanoTime());
             }
         }
     }