XmlUtils#readThisMapXml should handle null keys.

Both readThisValueXml & writeThisValueXml have been
implemented to handle values with "null" names.

Change readThisMapXml to allow such names as well, and
map them to the null key in the resulting HashMap.
readThisListXml and readThisSetXml already support
null names.

bug: https://code.google.com/p/android/issues/detail?id=63463

Change-Id: I1c93976656e45e7733113eb67d29b6bad5d25f1f