Map platform/external/autotest to external/autotest

Bug: 25813295
Test: syncs just fine
Change-Id: Idb1706c65d9d8b8bf965c570c762350b580a1f3f

default.xml

@@ -73,6 +73,7 @@
   <project path="external/apache-harmony" name="platform/external/apache-harmony" groups="pdk-cw-fs,pdk-fs" />
   <project path="external/apache-http" name="platform/external/apache-http" groups="pdk" />
   <project path="external/apache-xml" name="platform/external/apache-xml" groups="pdk" />
+  <project path="external/autotest" name="platform/external/autotest" />
   <project path="external/avahi" name="platform/external/avahi" />
   <project path="external/bison" name="platform/external/bison" groups="pdk" />
   <project path="external/blktrace" name="platform/external/blktrace" groups="pdk-cw-fs,pdk-fs" />