Android Gradle Plugin 3.x integration

[itoys]

Includes back compatibility for old RN versions
Resolves #1214

[rexih]

i'm an android developer, our team need to use code push and i'm studying the codepush.gradle file,
i check the commit said

• AAPT2-compatible Android build process integration
• enhancement
• enhancement
• fix assembleDebug

it was ambiguous for me,
could you tell me the difference of the two conditional branch :
if (variant.hasProperty("bundleJsAndAssets")) { } else { }
i found if has the property ,only create generateBundledResourcesHash task, if not ,it will create generateBundledResourcesHash and recordFilesBeforeBundleCommand task

why there were two different behavor