bugreport: fix warning.

The command the warning wants you to use instead is "bugreportz", not
"bugreport". You're already running bugreport!

...unless you're on the host and you're running `adb bugreport` which
silently uses bugreportz behind the scenes. No wonder humans are
confused!

Also error out if given any arguments, since no arguments are supported.

Bug: http://b/156667896
Bug: http://b/29448020
Test: adb shell bugreport
Test: adb shell bugreport --help
Change-Id: Ia141e89eb5019d52247f57c5095288e96f57dccd