android: Migrate to the Gradle build system

This uses a manual way to trigger the NDK build (the default with
on-the-fly Android.mk files does not work for us).

src/frontends/android/.classpath

@@ -1,9 +0,0 @@
-<?xml version="1.0" encoding="UTF-8"?>
-<classpath>
-	<classpathentry kind="con" path="com.android.ide.eclipse.adt.ANDROID_FRAMEWORK"/>
-	<classpathentry exported="true" kind="con" path="com.android.ide.eclipse.adt.LIBRARIES"/>
-	<classpathentry exported="true" kind="con" path="com.android.ide.eclipse.adt.DEPENDENCIES"/>
-	<classpathentry kind="src" path="src"/>
-	<classpathentry kind="src" path="gen"/>
-	<classpathentry kind="output" path="bin/classes"/>
-</classpath>

src/frontends/android/.gitignore

@@ -1,4 +1,7 @@
-bin/
-gen/
-libs/
-obj/
+.gradle/
+.idea/
+app/build/
+app/src/main/libs
+app/src/main/obj
+*.iml
+local.properties

src/frontends/android/README.ndk

@@ -1,5 +1,5 @@
-To build this within the NDK several things have to be added in the jni
-folder:
+To build this within the NDK several things have to be added in the
+app/src/main/jni/ folder:
 
  - strongswan: The strongSwan sources.  This can either be an extracted tarball,
    or a symlink to the Git repository.  To build from the repository the sources

src/frontends/android/app/build.gradle

@@ -0,0 +1,42 @@
+apply plugin: 'com.android.application'
+
+android {
+    compileSdkVersion 21
+    buildToolsVersion "22.0.1"
+
+    defaultConfig {
+        applicationId "org.strongswan.android"
+        minSdkVersion 15
+        targetSdkVersion 22
+        versionCode 28
+        versionName "1.5.0"
+    }
+
+    sourceSets.main {
+        jni.srcDirs = [] // disables the default ndk-build call (with on-the-fly Android.mk files)
+        jniLibs.srcDir 'src/main/libs'
+    }
+
+    task buildNative(type: Exec) {
+        workingDir 'src/main/jni'
+        commandLine "${android.ndkDirectory}/ndk-build", '-j', Runtime.runtime.availableProcessors()
+    }
+
+    task cleanNative(type: Exec) {
+        workingDir 'src/main/jni'
+        commandLine "${android.ndkDirectory}/ndk-build", 'clean'
+    }
+
+    tasks.withType(JavaCompile) {
+        compileTask -> compileTask.dependsOn buildNative
+    }
+
+    clean.dependsOn 'cleanNative'
+
+    buildTypes {
+        release {
+            minifyEnabled false
+            proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.txt'
+        }
+    }
+}

src/frontends/android/app/src/main/AndroidManifest.xml

@@ -16,11 +16,7 @@
     for more details.
 -->
 <manifest xmlns:android="http://schemas.android.com/apk/res/android"
-    package="org.strongswan.android"
-    android:versionCode="28"
-    android:versionName="1.5.0" >
-
-    <uses-sdk android:minSdkVersion="15" android:targetSdkVersion="22" />
+    package="org.strongswan.android">
 
     <uses-permission android:name="android.permission.INTERNET" />
     <uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />