Fix android build

android/build.gradle

@@ -1,23 +1,31 @@
+def safeExtGet(prop, fallback) {
+    rootProject.ext.has(prop) ? rootProject.ext.get(prop) : fallback
+}
 
 buildscript {
-    repositories {
-        jcenter()
-    }
-
-    dependencies {
-        classpath 'com.android.tools.build:gradle:1.3.1'
+    // The Android Gradle plugin is only required when opening the android folder stand-alone.
+    // This avoids unnecessary downloads and potential conflicts when the library is included as a
+    // module dependency in an application project.
+    if (project == rootProject) {
+        repositories {
+            google()
+            jcenter()
+        }
+        dependencies {
+            classpath 'com.android.tools.build:gradle:3.5.0'
+        }
     }
 }
 
 apply plugin: 'com.android.library'
 
 android {
-    compileSdkVersion 23
-    buildToolsVersion "23.0.1"
+    compileSdkVersion safeExtGet('compileSdkVersion', 28)
+    buildToolsVersion safeExtGet('buildToolsVersion', '28.0.3')
 
     defaultConfig {
-        minSdkVersion 16
-        targetSdkVersion 22
+        minSdkVersion safeExtGet('minSdkVersion', 19)
+        targetSdkVersion safeExtGet('targetSdkVersion', 28)
         versionCode 1
         versionName "1.0"
     }
@@ -27,10 +35,14 @@ android {
 }
 
 repositories {
-    mavenCentral()
+    mavenLocal()
+    maven {
+        // All of React Native (JS, Obj-C sources, Android binaries) is installed from npm
+        url "$rootDir/../node_modules/react-native/android"
+    }
 }
 
 dependencies {
-    compile 'com.facebook.react:react-native:+'
+    implementation 'com.facebook.react:react-native:+'  // From node_modules
 }
-
+