Add Bazel staging mode

This is to test modules that are near-prod ready.
Test: m nothing
Test: m nothing --bazel-mode-staging

Change-Id: I4f1f28862548a668fb7401ca1abe5c8a46a1b7e2
diff --git a/ci/mixed_droid.sh b/ci/mixed_droid.sh
index 9a5538a..1e13e4a 100755
--- a/ci/mixed_droid.sh
+++ b/ci/mixed_droid.sh
@@ -20,8 +20,10 @@
   MIXED_BUILD_FLAG="--bazel-mode-dev"
 elif [ "$MIXED_DROID_MODE" == "prod" ]; then
   MIXED_BUILD_FLAG="--bazel-mode"
+elif [ "$MIXED_DROID_MODE" == "staging" ]; then
+  MIXED_BUILD_FLAG="--bazel-mode-staging"
 else
-  echo "MIXED_DROID_MODE value \"$MIXED_DROID_MODE\" invalid. Must be either 'dev' or 'prod'"
+  echo "MIXED_DROID_MODE value \"$MIXED_DROID_MODE\" invalid. Must be either 'dev', 'prod', or 'staging'"
   exit 1
 fi