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: Iad90e67a2cdde86aabb95585975be56fc33361fe

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

@@ -31,7 +31,7 @@ public class StatsLogWriter implements LogWriter {
                 attribution,
                 SettingsEnums.PAGE_VISIBLE /* action */,
                 pageId,
-                null /* changedPreferenceKey */,
+                "" /* changedPreferenceKey */,
                 0 /* changedPreferenceIntValue */);
     }
 
@@ -41,7 +41,7 @@ public class StatsLogWriter implements LogWriter {
                 SettingsEnums.PAGE_UNKNOWN /* attribution */,
                 SettingsEnums.PAGE_HIDE /* action */,
                 pageId,
-                null /* changedPreferenceKey */,
+                "" /* changedPreferenceKey */,
                 0 /* changedPreferenceIntValue */);
     }