No html_dir_files if no LOCAL_DROIDDOC_HTML_DIR.
No html_dir_files if no LOCAL_DROIDDOC_HTML_DIR.

This removes unnecessary dependency if there is no
LOCAL_DROIDDOC_HTML_DIR.

Change-Id: If9e06b6e478a9cd61f5441aa376d996857ee2fb5

xorware/android_build,xorware/android_build,xorware/android_build,xorware/android_build,xorware/android_build