Fix optional_subdirs globbing

Bypassing c.glob() and using filepath.Glob() directly for non-glob
paths does not add dependencies on directories that contain missing
files.  For optional_subdirs, this means no dependency is added to
rerun the primary builder when an Android.bp file is added to an
optional_subdirs directory.  Always use c.glob(), for the non-optional
case it will not insert any dependencies if the file exists (as tested
by glob_test.go's no-wild tests), and if the file doesn't exist the
len(matches) == 0 will error out.

Change-Id: I370479c6e89f5ff590897702e256256a4dca6952