Create $(zip_root)/PVMFW & archive prebuilt used

It might be useful to have access to the prebuilt that the Android build
used to create the pvmfw partition so add a new directory (similarly to
other partitions) containing the prebuilt (which is also the only
content of the partition). This is particularly useful in ATC, when
detecting if the prebuilt was updated by the build.

Bug: 232253742
Test: m
Change-Id: I1116d91ffb073821568bcaeed30bbb43873dcafa