Add --dir option to findleaves.py

findleaves.py needs to be able to search multiple directories for
multiple filenames (Android.bp and Android.mk).  Add support for a new
--dir= option to specify one or more directories to search, and then
treat any remaining options as filenames.  If no directories are
specified, fall back to treating only the last option as a filename and
the rest as directories.

Change-Id: Ie879facd49192c6b4fafae070ad00eb21b1faa32