Allow app/build.gradle to take project properties when needed

android/build.gradle

 apply plugin: 'com.android.library'
 
+def DEFAULT_COMPILE_SDK_VERSION             = 26
+def DEFAULT_BUILD_TOOLS_VERSION             = "26.0.1"
+def DEFAULT_TARGET_SDK_VERSION              = 26
+def DEFAULT_GOOGLE_PLAY_SERVICES_VERSION    = "18.+"
+
 android {
-    compileSdkVersion 25
-    buildToolsVersion "25.0.2"
+    compileSdkVersion project.hasProperty('compileSdkVersion') ? project.compileSdkVersion : DEFAULT_COMPILE_SDK_VERSION
+    buildToolsVersion project.hasProperty('buildToolsVersion') ? project.buildToolsVersion : DEFAULT_BUILD_TOOLS_VERSION
 
     defaultConfig {
         minSdkVersion 16
-        targetSdkVersion 25
+        targetSdkVersion project.hasProperty('buildToolsVetargetSdkVersionrsion') ? project.buildToolsVersion : DEFAULT_TARGET_SDK_VERSION
         versionCode 1
         versionName "1.0"
     }
@@ -17,10 +22,12 @@ repositories {
 }
 
 dependencies {
+    def googlePlayServicesVersion = project.hasProperty('googlePlayServicesVersion') ? project.googlePlayServicesVersion : DEFAULT_GOOGLE_PLAY_SERVICES_VERSION
+
     compile fileTree(include: ['*.jar'], dir: 'libs')
     compile 'com.facebook.react:react-native:+'
-    compile 'com.google.firebase:firebase-core:11.+'
-    compile 'com.google.firebase:firebase-messaging:11.+'
+    compile "com.google.firebase:firebase-core:$googlePlayServicesVersion"
+    compile "com.google.firebase:firebase-messaging:$googlePlayServicesVersion"
     compile 'me.leolin:ShortcutBadger:1.1.17@aar'
 }