Fix update_engine libchrome log tag.

Test: boot and inspect logcat
Fixes: 158030597
Change-Id: I931af4faf6219ce42ff94080ea834721726dc0ba
diff --git a/logging_android.cc b/logging_android.cc
index 88b068b..0219075 100644
--- a/logging_android.cc
+++ b/logging_android.cc
@@ -241,8 +241,8 @@
   ignore_result(android::base::ConsumeSuffix(&sv, "\n"));
   std::string str(sv.data(), sv.size());
   // This will eventually be redirected to CombinedLogger.
-  // |tag| is ignored by CombinedLogger, so just leave it empty.
-  __android_log_write(priority, "" /* tag */, str.c_str());
+  // Use nullptr as tag so that liblog infers log tag from getprogname().
+  __android_log_write(priority, nullptr /* tag */, str.c_str());
   return true;
 }