Ignore JdkObsolete issue with StringBuffer

HttpServletRequest#getRequestURL() returns the obsolete StringBuffer
class. Add a util method to get the requets URL as String that allows us
to ignore the JdkObsolete issue.

Bug: Issue 15070
Signed-off-by: Edwin Kempin <ekempin@google.com>
Change-Id: I0dad2e4c2276088b7fffdfadfca7f24c553f5fd2