android: run the React packager when running from AS

When running the app from Android Studio the React packager is not automatically
started. In vanilla RN projects this is done by the "react-native run-android"
command, but often times it is desired to run from Android Studio.

This fixes that by starting the packager from Gradle.

android/scripts/run-packager.sh

@@ -0,0 +1,24 @@
+#!/bin/bash
+
+# This script is executed bt Gradle to start the React packager for Debug
+# targets.
+
+THIS_DIR=$(cd -P "$(dirname "$(readlink "${BASH_SOURCE[0]}" || echo "${BASH_SOURCE[0]}")")" && pwd)
+
+export RCT_METRO_PORT="${RCT_METRO_PORT:=8081}"
+echo "export RCT_METRO_PORT=${RCT_METRO_PORT}" > "${THIS_DIR}/../../node_modules/react-native/scripts/.packager.env"
+
+if nc -w 5 -z localhost ${RCT_METRO_PORT} ; then
+  if ! curl -s "http://localhost:${RCT_METRO_PORT}/status" | grep -q "packager-status:running" ; then
+    echo "Port ${RCT_METRO_PORT} already in use, packager is either not running or not running correctly"
+    exit 2
+  fi
+else
+    CMD="${THIS_DIR}/../../node_modules/react-native/scripts/launchPackager.command"
+    if [[ `uname` == "Darwin"  ]]; then
+        open -g "${CMD}" || echo "Can't start packager automatically"
+    else
+        xdg-open "${CMD}" || echo "Can't start packager automatically"
+    fi
+fi
+