Add platform_tests to mixed_droid.

This is to better match soong-only targets in bazel ci branch.

Bug: 193672919
Test: build/bazel/ci/mixed_droid.sh
Change-Id: I1e924d99ec73959dbbe03ea95031f634b530f8be
diff --git a/ci/mixed_droid.sh b/ci/mixed_droid.sh
index 002afc5..f3c0837 100755
--- a/ci/mixed_droid.sh
+++ b/ci/mixed_droid.sh
@@ -9,7 +9,7 @@
 fi
 
 # Run a mixed build of "droid"
-build/soong/soong_ui.bash --make-mode USE_BAZEL_ANALYSIS=1 BAZEL_STARTUP_ARGS="--max_idle_secs=5" BAZEL_BUILD_ARGS="--color=no --curses=no --show_progress_rate_limit=5" TARGET_PRODUCT=aosp_arm64 TARGET_BUILD_VARIANT=userdebug droid dist DIST_DIR=$DIST_DIR
+build/soong/soong_ui.bash --make-mode USE_BAZEL_ANALYSIS=1 BAZEL_STARTUP_ARGS="--max_idle_secs=5" BAZEL_BUILD_ARGS="--color=no --curses=no --show_progress_rate_limit=5" TARGET_PRODUCT=aosp_arm64 TARGET_BUILD_VARIANT=userdebug droid platform_tests dist DIST_DIR=$DIST_DIR
 
 # Verify there are artifacts under the out directory that originated from bazel.
 echo "Verifying OUT_DIR contains bazel-out..."