Mark implementation variant of prebuilts with stubs as not installable

This is necessary to avoid installing them. Previously, when install
dependencies were resolved in make, they weren't installed because they
got a .bootstrap suffix in their mk modules. However when that logic
moved into Soong by resolving transitive dependencies in
computeInstallDeps, they started to get dependencies and hence their
stubs could get installed, e.g. system/lib{,64}/libdexfile.so from
prebuilt_libdexfile.

Test: m nothing
Test: env NINJA_ARGS="-t path droid out/target/product/vsoc_x86_64/system/lib64/libdexfile.so" \
        m SOONG_CONFIG_art_module_source_build=false nothing
  verify that ninja reports no dependency path
Bug: 211770050
Bug: 220898484
Change-Id: Ifbfe31a15428926ce57b9e91b535b7ae79038fbd