init: report shutdown,container for sigterm of init

Change HandleSigtermSignal() handler to report shutdown,container. Add
the new reason to bootstat.  Remove log stutter as
HandlPowerctlMessage will also do a LOG(INFO) reporting
shutdown,container as reason.

Sending SIGTERM to init is to allow a host OS to ask an Android
Container instance to shutdown.  The temptation is to report
shutdown,sigterm but that does not accurately describe the usage
scenario.

Test: compile
Bug: 63736262
Change-Id: I3c5798921bdbef5d2689ad22a2e8103741b570b4
diff --git a/init/init.cpp b/init/init.cpp
index 817b33e..dbe833f 100644
--- a/init/init.cpp
+++ b/init/init.cpp
@@ -417,8 +417,7 @@
         return;
     }
 
-    LOG(INFO) << "Handling SIGTERM, shutting system down";
-    HandlePowerctlMessage("shutdown");
+    HandlePowerctlMessage("shutdown,container");
 }
 
 static void InstallSigtermHandler() {