Fix parsing of extension string

Previously the parsing found the next space and then added the the difference
between the current position and space to the set of tokens. This improperly
generated empty strings if there were consecutive spaces or if spaces existed at
the beginning or end of strings. To fix this, the parse is modified to use
simple stringstream parsing.

Bug: 22709246

Change-Id: I9e32c07bbf984eadccdadf1dc34437fa0c46088b