ota_from_target_files requires apexd_host

apexd_host is invoked in check_target_files_vintf.py
to prepare the apex directory before invoking checkvintf. ota_from_target_files uses check_target_files_vintf.py via (python_library) releasetools_check_target_files_vintf.

Since soong doesn't inherit "required:" property from library to binary, adding the dependency manually to ota_from_target_files.

Bug: n/a
Test: $ m ota_from_target_files # should build apexd_host
Change-Id: Ie7e6c3acf376c23960f242294f5a9bf99fec26aa