Temporarily enable use pvmfw prebuilts as fallback

This change add a temporary method of delivering pvmfw for x86_64. If
pvmfw building is not available (effectively on target other then
aarch64) check for prebuilt and if available use it instead. The
prebuilt is provided using three new `PRODUCT_PVMFW_IMAGE_PREBUILT`,
`PRODUCT_PVMFW_BIN_PREBUILT` and `PRODUCT_PVMFW_EMBEDDED_AVBKEY_PREBUILT`,
make variables. One for providing the module with signed pvmfw image,
one for raw pvmfw binary and one for specifing the embedded avb key
present in the prebuilt image.

If `PRODUCT_PVMFW_EMBEDDED_AVBKEY_PREBUILT` is absent it's assumed that
prebuilt image has default AOSP key embedded in it.

Bug: 391159299
Test: m
Change-Id: If4dfb9099d000ac44cda2fd9ccd0892767b4f066