Use ISO 8601 format for LocalLog local timestamps

Change to using ISO 8601 duration formatting for LocalLog local
timestamps, which makes it consistent with other time / time zone
detection dumpsys for easier comparison.

LocalLog is used by NetworkStack code as a standalone library, so
therefore has to depend on public SDK APIs, hence the use of Duration.

The local timestamp logging mode is only currently used for time / time
zone detection, which is low-frequency logging.

Bug: 204855374
Test: atest core/tests/coretests/src/android/util/LocalLogTest.java
Change-Id: Idce0d39c59670facdf48deca852b862ff0d71991

core/java/android/util/LocalLog.java

@@ -22,6 +22,7 @@ import android.os.SystemClock;
 
 import java.io.FileDescriptor;
 import java.io.PrintWriter;
+import java.time.Duration;
 import java.time.Instant;
 import java.time.LocalDateTime;
 import java.util.ArrayDeque;
@@ -63,7 +64,8 @@ public final class LocalLog {
         if (mUseLocalTimestamps) {
             logLine = LocalDateTime.now() + " - " + msg;
         } else {
-            logLine = SystemClock.elapsedRealtime() + " / " + Instant.now() + " - " + msg;
+            logLine = Duration.ofMillis(SystemClock.elapsedRealtime())
+                    + " / " + Instant.now() + " - " + msg;
         }
         append(logLine);
     }