MetricId: Avoid byte[] and string ambiguity

Passing null to the function will result in conflict with Bluetooth proto
(int int int byte[] int)
Bug: 122571213
Test: Build

Change-Id: I2d1c3896d5d850a81a987aded14e3f0c05558e8c

src/com/android/settings/core/instrumentation/StatsLogWriter.java

@@ -30,7 +30,7 @@ public class StatsLogWriter implements LogWriter {
                 attribution, /* from pageId */
                 SettingsEnums.PAGE_VISIBLE /* action */,
                 pageId, /* target pageId */
-                null /* changedPreferenceKey */,
+                "" /* changedPreferenceKey */,
                 latency /* changedPreferenceIntValue */);
     }
 
@@ -40,7 +40,7 @@ public class StatsLogWriter implements LogWriter {
                 SettingsEnums.PAGE_UNKNOWN /* attribution */,
                 SettingsEnums.PAGE_HIDE /* action */,
                 pageId,
-                null /* changedPreferenceKey */,
+                "" /* changedPreferenceKey */,
                 visibleTime /* changedPreferenceIntValue */);
     }