Support for `prebuilt_avb` in Soong

Generally, the `root` directories for `prebuilt_etc` and
`prebuilt_root` are <partitions>. And <partitions> default to false
for InstallInRoot(), they always create a "system" subdirectory.
(See android/paths.go:modulePartition())

However, avb keys like `q-developer-gsi.avbpubkey` are installed by
default in the `avb` subdirectory under the `root directory` of the
<partition>. Therefore, it is necessary to create `prebuilt_avb` in
Soong and force the installation directory to be the `avb` subdirectory
under the `root directory` of the <partition> to meet the requirements.

Bug: 347636127
Test: go test -run TestPrebuiltAvbInstallDirPath
Change-Id: I0ee08492da9a96a567700a7285c527b280dcd5db