ANDROID_SDK_ROOT support (#2106)

Summary:
According to Google, ANDROID_HOME is deprecated.
This means, that having only ANDROID_SDK_ROOT as environment variable leading to Android SDK is enough.
Flipper supports only ANDROID_HOME, so I would like to add ANDROID_SDK_ROOT support.
However, there is a set of rules of ANDROID_HOME and ANDROID_SDK_ROOT priority.
Everything is mentioned on a this page: https://developer.android.com/studio/command-line/variables

## Changelog
Added ANDROID_SDK_ROOT environment variable support

Pull Request resolved: https://github.com/facebook/flipper/pull/2106

Test Plan: Probably I would need a help with this

Reviewed By: mweststrate

Differential Revision: D27505024

Pulled By: passy

fbshipit-source-id: 8bdc66a9652c673fd44944d87af1efc9e93d0f1b

desktop/app/src/dispatcher/androidDevice.tsx

@@ -138,7 +138,12 @@ export default (store: Store, logger: Logger) => {
   const watchAndroidDevices = () => {
     // get emulators
     promisify(which)('emulator')
-      .catch(() => `${process.env.ANDROID_HOME || ''}/tools/emulator`)
+      .catch(
+        () =>
+          `${
+            process.env.ANDROID_HOME || process.env.ANDROID_SDK_ROOT || ''
+          }/tools/emulator`,
+      )
       .then((emulatorPath) => {
         child_process.exec(
           `${emulatorPath} -list-avds`,