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/chrome/DevicesButton.js

@@ -21,14 +21,12 @@ type Props = {
 
 class DevicesButton extends Component<Props> {
   launchEmulator = (name: string) => {
-    const child = spawn(
-      `${process.env.ANDROID_HOME || ''}/tools/emulator`,
-      [`@${name}`],
-      {
-        detached: true,
-        stdio: 'ignore',
-      },
-    );
+    const child = spawn('emulator', [`@${name}`], {
+      detached: true,
+    });
+    child.stderr.on('data', data => {
+      console.error(`Android emulator error: ${data}`);
+    });
     child.on('error', console.error);
     this.props.preferDevice(name);
   };

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');

static/index.js

@@ -23,6 +23,12 @@ if (!process.env.ANDROID_HOME) {
   process.env.ANDROID_HOME = '/opt/android_sdk';
 }
 
+// emulator/emulator is more reliable than tools/emulator, so prefer it if
+// it exists
+process.env.PATH = `${process.env.ANDROID_HOME}/emulator:${
+  process.env.ANDROID_HOME
+}/tools:${process.env.PATH}`;
+
 if (process.platform === 'darwin') {
   // If we are running on macOS and the app is called Flipper, we add a comment
   // with the old name, to make it findable via Spotlight using its old name.