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