Fix emulator launching bug

Summary:
I was getting the following error when trying to launch an emulator:
`PANIC: Missing emulator engine program for 'x86' CPU.`

It seems like emulator/emulator is more reliable than tools/emulator: https://stackoverflow.com/questions/26483370/android-emulator-error-message-panic-missing-emulator-engine-program-for-x86

So put them both on the path in that order.

Reviewed By: passy

Differential Revision: D13040325

fbshipit-source-id: 11e8b24b2a99e02955128d5fb7c17764b98388fa

src/dispatcher/androidDevice.js

@@ -59,7 +59,7 @@ export default (store: Store, logger: Logger) => {
   const watchAndroidDevices = () => {
     // get emulators
     child_process.exec(
-      '$ANDROID_HOME/tools/emulator -list-avds',
+      'emulator -list-avds',
       (error: ?Error, data: ?string) => {
         if (error != null || data == null) {
           console.error(error || 'Failed to list AVDs');