Add autotest repository

Bug: 26233392
Test: Copied this line from internal branch
diff --git a/default.xml b/default.xml
index 67184b6..31d5bf2 100644
--- a/default.xml
+++ b/default.xml
@@ -45,6 +45,7 @@
   <project path="device/generic/x86_64" name="device/generic/x86_64" groups="bdk" />
   <project path="external/aac" name="platform/external/aac" groups="bdk" />
   <project path="external/avahi" name="platform/external/avahi" groups="bdk" />
+  <project path="external/autotest" name="platform/external/autotest" groups="bdk" />
   <project path="external/bison" name="platform/external/bison" groups="bdk" />
   <project path="external/boringssl" name="platform/external/boringssl" groups="bdk" />
   <project path="external/bouncycastle" name="platform/external/bouncycastle" groups="bdk" />