configurable devEnabled in React.gradle

Summary:
This PR adds support for configurable devEnabled option when building an android app. This is currently hardcoded.

The reason for making this configurable is this: I have an app that uses code-push and 3 buildConfigs. I want to have a debugging version which has `devEnabled = true`, then a staging version which has `devEnabled = false` (this version of the app is used internally for testing and should behave just like a release version of the app, and when the tests succeed the changes are promoted into the release version to the users out there, using code-push). The last version is a standard release version with `devEnabled = false`.

Currently, `devEnabled` is hardwired like this: `!targetName.toLowerCase().contains("release")` so by default my `staging` buildConfig will have `devEnabled = true` but I'd like it to be false.

With this PR it'd be possible to configure this as follows, while not breaking the current behavior.
`'devDisabledIn${productFlavor}${buildType}'`
`'devDisabledIn${buildType
Closes https://github.com/facebook/react-native/pull/11438

Differential Revision: D4630513

Pulled By: hramos

fbshipit-source-id: b6817cf4c144fc948f76785e9cb5f93a13a6a6a2

local-cli/templates/HelloWorld/android/app/build.gradle

@@ -33,6 +33,13 @@ import com.android.build.OutputFile
  *   // bundleInPaidRelease: true,
  *   // bundleInBeta: true,
  *
+ *   // whether to disable dev mode in custom build variants (by default only disabled in release)
+ *   // for example: to disable dev mode in the staging build type (if configured)
+ *   devDisabledInStaging: true,
+ *   // The configuration property can be in the following formats
+ *   //         'devDisabledIn${productFlavor}${buildType}'
+ *   //         'devDisabledIn${buildType}'
+ *
  *   // the root of your project, i.e. where "package.json" lives
  *   root: "../../",
  *