Summary:
If for instance you're using a custom transformer, having this hook will let you specify it without forking all of react.gradle
**Test plan:** Specified some additional packager args in my app's `android/app/build.gradle`:
```groovy
project.ext.react = [
bundleInDebug: true,
extraPackagerArgs: ["--transformer", "path/to/my/transformer.js"]
]
```
and ensured they show up when gradle invokes the bundler.
Closes https://github.com/facebook/react-native/pull/7858
Differential Revision: D3382996
Pulled By: mkonicek
fbshipit-source-id: 437b2e6c902931d45b9d2f7ec97c833ba0cd3217
1f2027a1fe