Android: allow app/build.gradle to specify how node gets invoked for the packager

Summary:
**Test plan:** With the given patch applied to `react.gradle`, I specified the following in my `android/app/build.gradle`:
```
project.ext.react = [
  nodeExecutableAndArgs: ["node", "--max_old_space_size=4096"]
]
```
and ensured in a `./gradlew installDebug --debug` run that the packager gets indeed invoked with these parameters.
Closes https://github.com/facebook/react-native/pull/7903

Differential Revision: D3390543

fbshipit-source-id: cf440b36633420b8f67070f47dfabf4c84cb28a7