Add generic board-agnostic pre-built pvmfw.img

Make use of the pvmfw.img-building AOSP framework to provide a common
default image instead of requiring boards to necessarily provide their
own but allow over-writing it through BOARD_PREBUILT_PVMFWIMAGE, if
desired.

Note: The Android Virtualization Framework requires the pvmfw partition.

Bug: 199717422
Test: m ${ANDROID_PRODUCT_OUT}/pvmfw.img
Change-Id: I5e06f08d6e22178e45254b7ab63acd3855d99a38