Generate boot images for host from prebuilts

Previously, when building from prebuilts boot no rules were created to
produce the boot image files for the host, i.e. the OS on which the
build was running. That caused problems with checkbuilds. No rules were
produced as there was no host variant of a prebuilt apex to provide
them.

This change restructures the code to allow the prebuilt bootclasspath
fragment to build the host variants of the files from the dex files
provided by the prebuilt APEX. The generated files will not be the same
as they would be if built from source as there is no boot image profile
to use but it should be sufficient to satisfy the checkbuild target and
allow for host side testing.

Bug: 192575099
Test: m SOONG_CONFIG_art_module_source_build=false droid dist checkbuild
Change-Id: I6af00f19bb71aa18dd462f5eac6aa38e3e721023