Atest: prebuilt from aosp_cf_x86_phone-userdebug.

The atest prebuilt binary is download from aosp_master
aosp_cf_x86_phone-userdebug 5174827.

Bug: b/118801769

Test: $ source build/envsetup.sh; lunch
      $ rm -f $ANDROID_HOST_OUT/bin/atest
      $ rm -f $ANDROID_BUILD_TOP/out/soong/host/*x86/bin/atest
      $ atest example/reboot -v

Change-Id: I6a41aade5c7eb1eac824afedc7041ee068d04982