Upgrade to expat 2.2.6.

From `Changes`:

  Release 2.2.6 Sun August 12 2018
        Bug fixes:
       #170 #206  Avoid doing arithmetic with NULL pointers in XML_GetBuffer
       #204 #205  Fix 2.2.5 regression with suspend-resume while parsing
                    a document like '<root/>'

        Other changes:
       #165 #168  Autotools: Fix docbook-related configure syntax error
            #166  Autotools: Avoid grep option `-q` for Solaris
            #167  Autotools: Support
                    ./configure DOCBOOK_TO_MAN="xmlto man --skip-validation"
       #159 #167  Autotools: Support DOCBOOK_TO_MAN command which produces
                    xmlwf.1 rather than XMLWF.1; also covers case insensitive
                    file systems
            #181  Autotools: Drop -rpath option passed to libtool
            #188  Autotools: Detect and deny SGML docbook2man as ours is XML
            #188  Autotools/CMake: Support command db2x_docbook2man as well
            #174  CMake: Introduce option WARNINGS_AS_ERRORS, defaults to OFF
       #184 #185  CMake: Introduce option MSVC_USE_STATIC_CRT, defaults to OFF
       #207 #208  CMake: Introduce option XML_UNICODE and XML_UNICODE_WCHAR_T,
                    both defaulting to OFF
            #175  CMake: Prefer check_symbol_exists over check_function_exists
            #176  CMake: Create the same pkg-config file as with GNU Autotools
       #178 #179  CMake: Use GNUInstallDirs module to set proper defaults for
                    install directories
            #208  CMake: Utilize expat_config.h.cmake for XML_DEV_URANDOM
            #180  Windows: Fix compilation of test suite for Visual Studio 2008
  #131 #173 #202  Address compiler warnings
  #187 #190 #200  Fix miscellaneous typos
                  Version info bumped from 7:7:6 to 7:8:6

        Special thanks to:
            Anton Maklakov
            Benjamin Peterson
            Brad King
            Franek Korta
            Frank Rast
            Joe Orton
            luzpaz
            Pedro Vicente
            Rainer Jung
            Rhodri James
            Rolf Ade
            Rolf Eike Beer
            Thomas Beutlich
            Tomasz Kłoczko

Bug: N/A
Test: cts-tradefed run cts -m CtsLibcoreTestCases -t libcore.xml.ExpatSaxParserTest
Change-Id: I39ea70944ada1553cbc721ff9fd710c1937c7244