Use project-wide properties and new dependency

* Support for gradle 3.+
* Support for RN 0.56.+

android/build.gradle

 apply plugin: 'com.android.library'
 
+def DEFAULT_COMPILE_SDK_VERSION   = 26
+def DEFAULT_BUILD_TOOLS_VERSION   = "26.0.3"
+def DEFAULT_TARGET_SDK_VERSION    = 26
+def DEFAULT_MIN_SDK_VERSION       = 16
+
 android {
-    compileSdkVersion 23
-    buildToolsVersion "23.0.1"
+        compileSdkVersion rootProject.hasProperty('compileSdkVersion') ? rootProject.compileSdkVersion : DEFAULT_COMPILE_SDK_VERSION
+        buildToolsVersion rootProject.hasProperty('buildToolsVersion') ? rootProject.buildToolsVersion : DEFAULT_BUILD_TOOLS_VERSION
 
-    defaultConfig {
-        minSdkVersion 16
-        targetSdkVersion 22
+  defaultConfig {
+        minSdkVersion rootProject.hasProperty('minSdkVersion') ? rootProject.minSdkVersion : DEFAULT_MIN_SDK_VERSION
+        targetSdkVersion rootProject.hasProperty('targetSdkVersion') ? rootProject.targetSdkVersion : DEFAULT_TARGET_SDK_VERSION
         versionCode 1
         versionName "1.0"
         ndk {
@@ -16,5 +21,5 @@ android {
 }
 
 dependencies {
-    compile 'com.facebook.react:react-native:+'
+    implementation 'com.facebook.react:react-native:+'
 }