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/build.gradle

@@ -0,0 +1,14 @@
+buildscript {
+    repositories {
+        jcenter()
+    }
+    dependencies {
+        classpath 'com.android.tools.build:gradle:1.3.0'
+    }
+}
+
+allprojects {
+    repositories {
+        jcenter()
+    }
+}