Bug 1037302 - Avoid excess string creation in WifiCommand.jsm's getConnectionInfoICS(). r=hchang.

This changes the code to use search() and indexOf() to find the boundaries of
the relevant values, and substring() to extract them. This reduces the number
of strings created on each invocation by 8x.

The patch changes the behaviour if a key appears more than once. With the old
code the last occurrence would be used. With the new code the first one is
used. Hopefully this doesn't matter.