Add real to elapsed offset to shell transition trace

So the entries can be mapped to the respective unix timestamps for Winscope.

Test: Capture trace from winscope and make sure timeline is properly synchronized
Bug: 277181336
Change-Id: I2478b3ce98d2ffe639276715b3c3984a4e450152

libs/WindowManager/Shell/proto/wm_shell_transition_trace.proto

@@ -37,6 +37,9 @@ message WmShellTransitionTraceProto {
     required fixed64 magic_number = 1;
     repeated Transition transitions = 2;
     repeated HandlerMapping handlerMappings = 3;
+    /* offset between real-time clock and elapsed time clock in nanoseconds.
+    Calculated as: 1000000 * System.currentTimeMillis() - SystemClock.elapsedRealtimeNanos() */
+    optional fixed64 real_to_elapsed_time_offset_nanos = 4;
 }
 
 message Transition {

libs/WindowManager/Shell/src/com/android/wm/shell/transition/Tracer.java

@@ -28,7 +28,6 @@ import android.os.Trace;
 import android.util.Log;
 
 import com.android.internal.util.TraceBuffer;
-import com.android.wm.shell.nano.HandlerMapping;
 import com.android.wm.shell.sysui.ShellCommandHandler;
 
 import com.google.protobuf.nano.MessageNano;
@@ -41,6 +40,7 @@ import java.util.ArrayList;
 import java.util.HashMap;
 import java.util.Map;
 import java.util.Queue;
+import java.util.concurrent.TimeUnit;
 
 /**
  * Helper class to collect and dump transition traces.
@@ -241,6 +241,10 @@ public class Tracer implements ShellCommandHandler.ShellCommandActionHandler {
                     new com.android.wm.shell.nano.WmShellTransitionTraceProto();
             proto.magicNumber = MAGIC_NUMBER_VALUE;
             writeHandlerMappingToProto(proto);
+            long timeOffsetNs =
+                    TimeUnit.MILLISECONDS.toNanos(System.currentTimeMillis())
+                            - SystemClock.elapsedRealtimeNanos();
+            proto.realToElapsedTimeOffsetNanos = timeOffsetNs;
             int pid = android.os.Process.myPid();
             LogAndPrintln.i(pw, "Writing file to " + file.getAbsolutePath()
                     + " from process " + pid);