Keyguard: Set WIDGET_CATEGORY_KEYGUARD when AppWidgetHostView is created

Allows the default lockscreen widget to inherit the keyguard widget
host category.  Without this, only widgets added through
KeyguardActivityLauncher are assigned the correct
OPTION_APPWIDGET_HOST_CATEGORY.

Change-Id: Ic97818d85ca5f00da0ce43bb36006d66a6935363