[Android] Implement a ViewPager for the settings. Also, move classes into appropriate packages to make things cleaner.

Source/Android/res/layout/prefs_viewpager.xml

@@ -0,0 +1,6 @@
+<android.support.v4.view.ViewPager xmlns:android="http://schemas.android.com/apk/res/android"
+    xmlns:tools="http://schemas.android.com/tools"
+    android:id="@+id/pager"
+    android:layout_width="match_parent"
+    android:layout_height="match_parent"
+    tools:context=".PrefsActivity" />