Fix prebuilt header files to produce proper AndroidMk entries.

Necessary for modules that depend on headers through
LOCAL_HEADER_LIBRARIES in legacy Android.mk files.

Test: m nothing
Test: m nothing SOONG_CONFIG_art_module_source_build=false
  on internal master
Test: m droid checkbuild SOONG_CONFIG_art_module_source_build=false
Test: m SOONG_CONFIG_art_module_source_build=false \
        out/soong/.intermediates/prebuilts/module_sdk/art/current/sdk/prebuilt_jni_headers/linux_glibc_x86_64/prebuilt_jni_headers.a
      ar t out/soong/.intermediates/prebuilts/module_sdk/art/current/sdk/prebuilt_jni_headers/linux_glibc_x86_64/prebuilt_jni_headers.a \
        && echo ok
Bug: 202192894
Change-Id: Ied267053d1fc3563870a3faaf41d79983a22815a