Add framework for building the pvmfw.img partition

Adapt the variables necessary for building pvmfw.img by following what
was done for other Android partitions and introducing:

 - PRODUCT_BUILD_PVMFW_IMAGE
 - BUILDING_PVMFW_IMAGE
 - BUILT_PVMFWIMAGE_TARGET

Replace the manual 'cp' by the more common 'copy-one-file'.

Bug: 199831815
Test: m ${ANDROID_PRODUCT_OUT}/pvmfw.img # with TARGET_PKVM_ENABLED=true
Change-Id: I5e4bbcbdbf4b96281ee54631938f097e9744883c