bump android gradle plugin to 3.4.0 (#24463)

Summary:
This PR bumps Android Gradle Plugin to 3.4.0, which enables R8 shrinker by default and improves build performance significantly.

Disabled R8 for ReactAndroid because it'll strip out AndroidX and other libraries bundled in ReactAndroid.

[Android] [Changed] - bump Android Gradle plugin to 3.4.0
Pull Request resolved: https://github.com/facebook/react-native/pull/24463

Differential Revision: D15107117

Pulled By: hramos

fbshipit-source-id: 35a03dc9955e889c9399faeaf9a862e0fc044fc4

# Conflicts:
#	ReactAndroid/gradle.properties
#	build.gradle
#	template/android/build.gradle

ReactAndroid/gradle.properties

@@ -21,3 +21,5 @@ DOUBLE_CONVERSION_VERSION=1.1.6
 FOLLY_VERSION=2018.10.22.00
 GLOG_VERSION=0.3.5
 JSC_VERSION=236355.1.1
+
+android.enableR8=false

build.gradle

@@ -10,7 +10,7 @@ buildscript {
         jcenter()
     }
     dependencies {
-        classpath("com.android.tools.build:gradle:3.3.0")
+        classpath("com.android.tools.build:gradle:3.4.0")
         classpath("de.undercouch:gradle-download-task:3.4.3")
 
         // NOTE: Do not place your application dependencies here; they belong

react.gradle

@@ -146,6 +146,11 @@ afterEvaluate {
                 into ("merged_assets/${variant.name}/merge${targetName}Assets/out") {
                     from(jsBundleDir)
                 }
+
+                // Workaround for Android Gradle Plugin 3.4+ new asset directory
+                into ("merged_assets/${variant.name}/out") {
+                    from(jsBundleDir)
+                }
             }
 
             // mergeAssets must run first, as it clears the intermediates directory

template/android/build.gradle

@@ -13,7 +13,7 @@ buildscript {
         jcenter()
     }
     dependencies {
-        classpath 'com.android.tools.build:gradle:3.3.0'
+        classpath("com.android.tools.build:gradle:3.4.0")
 
         // NOTE: Do not place your application dependencies here; they belong
         // in the individual module build.gradle files