Close DirectoryStream in ActivityThread after use.

Right now, the directory stream is never closed. This leads to confusing
"A resource failed to call close" errors. After some debugging, they
seem to be coming from sun.nio.fs.UnixSecureDirectoryStream objects and
reported here:
https://cs.android.com/android/platform/superproject/+/master:libcore/dalvik/src/main/java/dalvik/system/CloseGuard.java;l=340;drc=e32570b11273e703580b60fc9d59b96223f376da.

Test: m dist -j && acloud create --local-instance --local-image
Change-Id: Ie9fddc2eab505527985d8c6b2822623fe16c7f89