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/bootstat/bootstat.cpp b/bootstat/bootstat.cpp
index 7c0b15e..17986b9 100644
--- a/bootstat/bootstat.cpp
+++ b/bootstat/bootstat.cpp
@@ -204,6 +204,7 @@
     {"reboot,shell", 66},
     {"reboot,adb", 67},
     {"reboot,userrequested", 68},
+    {"shutdown,container", 69},  // Host OS asking Android Container to shutdown
 };
 
 // Converts a string value representing the reason the system booted to an