android: Update Gradle plugin and wrapper

src/frontends/android/app/build.gradle

@@ -2,7 +2,7 @@ apply plugin: 'com.android.application'
 
 android {
     compileSdkVersion 23
-    buildToolsVersion '25.0.0'
+    buildToolsVersion '27.0.3'
 
     defaultConfig {
         applicationId "org.strongswan.android"
@@ -43,7 +43,7 @@ android {
 }
 
 dependencies {
-    compile 'com.android.support:appcompat-v7:23.3.0'
+    implementation 'com.android.support:appcompat-v7:23.3.0'
-    compile 'com.android.support:design:23.3.0'
+    implementation 'com.android.support:design:23.3.0'
-    testCompile 'junit:junit:4.12'
+    testImplementation 'junit:junit:4.12'
 }

src/frontends/android/build.gradle

@@ -1,14 +1,16 @@
 buildscript {
     repositories {
         jcenter()
+        google()
     }
     dependencies {
-        classpath 'com.android.tools.build:gradle:2.3.3'
+        classpath 'com.android.tools.build:gradle:3.1.3'
     }
 }
 
 allprojects {
     repositories {
         jcenter()
+        google()
     }
 }

src/frontends/android/gradle/wrapper/gradle-wrapper.properties

@@ -1,6 +1,6 @@
-#Tue Apr 18 15:57:06 CEST 2017
+#Mon Jun 04 11:56:43 CEST 2018
 distributionBase=GRADLE_USER_HOME
 distributionPath=wrapper/dists
 zipStoreBase=GRADLE_USER_HOME
 zipStorePath=wrapper/dists
-distributionUrl=https\://services.gradle.org/distributions/gradle-3.3-all.zip
+distributionUrl=https\://services.gradle.org/distributions/gradle-4.4-all.zip