Android buildbot: Add missing @@@ echo to 'bb_run_unit_tests'

Without this, a failure in this function is mistakenly reported
as a failure in the previous bb_xxxxx function called. For example,
see the following link where the failure is reported in
'bb_reboot_phones' when it should not:

http://build.chromium.org/p/chromium.linux/builders/Android%20Tests/builds/23/steps/Rebooting%20phones/logs/stdio

BUG=none
NOTRY=true
TBR=yfriedman@chromium.org,ilevy@chromium.org


Review URL: https://chromiumcodereview.appspot.com/11648009

git-svn-id: http://src.chromium.org/svn/trunk/src/build@173895 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

android/buildbot/buildbot_functions.sh

@@ -220,6 +220,7 @@ function bb_print_logcat {
 
 # Run tests on an actual device.  (Better have one plugged in!)
 function bb_run_unit_tests {
+  echo "@@@BUILD_STEP Run unit tests on device@@@"
   build/android/run_tests.py --xvfb --verbose
 }