Add support for split build per architecture

Summary:
This allows everyone to deploy significantly smaller APKs to they Play Store by building separate APKs for ARM, x86 architectures.

For a simple app, a release APK minified with Produard:
- Universal APK is **7MB**
- x86 APK is **4.6MB** (34% reduction)
- ARM APK is **3.7MB** (47% reduction)

Created a sample project, uncommented `// include "armeabi-v7a", 'x86'`:

    cd android
    ./gradlew assembleDebug

Three APKs were created, unzipped each: one has only x86 binaries,
one has ARM binaries, one has both.

    ./gradlew assembleRelease

Three APKs were created, JS bundle is correcly added to assets.

    react-native run-android

The correct APK is installed on the emulator and the app runs fine
(Gradle output: "Installing APK 'app-x86-debug.apk'").

With the line commented out the behavior is exactly the same as before,
only one universal APK is built.

Checked that version codes are set correctly as described in
http://tools.android.com/tech-docs/new-build-system/user-guide/apk-splits
http://developer.android.com/intl/ru/google/play/publishing/multiple-apks.html

Closes facebook/react-native#5160

Reviewed By: svcscm

Differential Revision: D2811443

Pulled By: mkonicek

fb-gh-sync-id: 97b22b9cd567e53b8adac36669b90768458b7a55

Author: Martin Konicek

generator-android/templates/src/app/build.gradle

@@ -1,5 +1,7 @@
 apply plugin: "com.android.application"
 
+import com.android.build.OutputFile
+
 /**
  * The react.gradle file registers two tasks: bundleDebugJsAndAssets and bundleReleaseJsAndAssets.
  * These basically call `react-native bundle` with the correct arguments during the Android build
@@ -49,6 +51,22 @@ apply plugin: "com.android.application"
 
 apply from: "react.gradle"
 
+/**
+ * Set this to true to create three separate APKs instead of one:
+ *   - A universal APK that works on all devices
+ *   - An APK that only works on ARM devices
+ *   - An APK that only works on x86 devices
+ * The advantage is the size of the APK is reduced by about 4MB.
+ * Upload all the APKs to the Play Store and people will download
+ * the correct one based on the CPU architecture of their device.
+ */
+def enableSeparateBuildPerCPUArchitecture = false
+
+/**
+ * Run Proguard to shrink the Java bytecode in release builds.
+ */
+def enableProguardInReleaseBuilds = true
+
 android {
     compileSdkVersion 23
     buildToolsVersion "23.0.1"
@@ -63,12 +81,33 @@ android {
             abiFilters "armeabi-v7a", "x86"
         }
     }
+    splits {
+        abi {
+            enable enableSeparateBuildPerCPUArchitecture
+            universalApk true
+            reset()
+            include "armeabi-v7a", "x86"
+        }
+    }
     buildTypes {
         release {
-            minifyEnabled false  // Set this to true to enable Proguard
+            minifyEnabled enableProguardInReleaseBuilds
             proguardFiles getDefaultProguardFile("proguard-android.txt"), "proguard-rules.pro"
         }
     }
+    // applicationVariants are e.g. debug, release
+    applicationVariants.all { variant ->
+        variant.outputs.each { output ->
+            // For each separate APK per architecture, set a unique version code as described here:
+            // http://tools.android.com/tech-docs/new-build-system/user-guide/apk-splits
+            def versionCodes = ["armeabi-v7a":1, "x86":2]
+            def abi = output.getFilter(OutputFile.ABI)
+            if (abi != null) {  // null for the universal-debug, universal-release variants
+                output.versionCodeOverride =
+                        versionCodes.get(abi) * 1048576 + defaultConfig.versionCode
+            }
+        }
+    }
 }
 
 dependencies {

generator-android/templates/src/app/react.gradle

@@ -74,14 +74,33 @@ task bundleReleaseJsAndAssets(type: Exec) {
     enabled config.bundleInRelease ?: true
 }
 
+void runBefore(String dependentTaskName, Task task) {
+    Task dependentTask = tasks.findByPath(dependentTaskName);
+    if (dependentTask != null) {
+        dependentTask.dependsOn task
+    }
+}
+
 gradle.projectsEvaluated {
+
     // hook bundleDebugJsAndAssets into the android build process
+
     bundleDebugJsAndAssets.dependsOn mergeDebugResources
     bundleDebugJsAndAssets.dependsOn mergeDebugAssets
-    processDebugResources.dependsOn bundleDebugJsAndAssets
+
+    runBefore('processArmeabi-v7aDebugResources', bundleDebugJsAndAssets)
+    runBefore('processX86DebugResources', bundleDebugJsAndAssets)
+    runBefore('processUniversalDebugResources', bundleDebugJsAndAssets)
+    runBefore('processDebugResources', bundleDebugJsAndAssets)
 
     // hook bundleReleaseJsAndAssets into the android build process
+
     bundleReleaseJsAndAssets.dependsOn mergeReleaseResources
     bundleReleaseJsAndAssets.dependsOn mergeReleaseAssets
-    processReleaseResources.dependsOn bundleReleaseJsAndAssets
+
+    runBefore('processArmeabi-v7aReleaseResources', bundleReleaseJsAndAssets)
+    runBefore('processX86ReleaseResources', bundleReleaseJsAndAssets)
+    runBefore('processUniversalReleaseResources', bundleReleaseJsAndAssets)
+    runBefore('processReleaseResources', bundleReleaseJsAndAssets)
+
 }