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
Initialize pointer members to NULL to avoid illegal reference

mBitmap and mTexture is not initialized to NULL which causes
illegal address access when it fails to be created from
oversized bitmap.
Cherry-picked from AOSP: https://android-review.googlesource.com/#/c/79320/

Change-Id: Iea54bec8788bc5f3a10040fdb43f416c0d41a14c