android: Update platform tools and pull in support libs

We'll have to change some stuff that Google deprecated (e.g. the tabs in
the ActionBar) and that requires changing the theme at least in activities.
Since that would look a bit inconsistent we'll change it globally and
use parts of the support library.

src/frontends/android/app/build.gradle

@@ -1,8 +1,8 @@
 apply plugin: 'com.android.application'
 
 android {
-    compileSdkVersion 21
-    buildToolsVersion "22.0.1"
+    compileSdkVersion 23
+    buildToolsVersion "23.0.3"
 
     defaultConfig {
         applicationId "org.strongswan.android"
@@ -42,5 +42,7 @@ android {
 }
 
 dependencies {
+    compile 'com.android.support:appcompat-v7:23.3.0'
+    compile 'com.android.support:design:23.3.0'
     testCompile 'junit:junit:4.12'
 }