Add animation samples (#118)

* initialize animations sample project

* add two demos, set up routing

* clean up expand_card demo

* update README

* clean up expand_card demo

* dartfmt

* update comment

* address code review comments

* swap AnimatedContainer and AnimatedCrossFade

* use AnimatedSwitcher instead of AnimatedCrossFade

* Revert "use AnimatedSwitcher instead of AnimatedCrossFade"

This reverts commit e112e02549.

* rename expanded -> selected

* use  Dart 2.4.0 constraint

* update README

* address code review comments

update images
add curves to expand_card
update pubspec.lock

* add @override annotation

* add animations project to travis script

* add empty test for travis

* add copyright notice to animations/ project

animations/android/app/src/main/res/drawable/launch_background.xml

@@ -0,0 +1,12 @@
+<?xml version="1.0" encoding="utf-8"?>
+<!-- Modify this file to customize your launch splash screen -->
+<layer-list xmlns:android="http://schemas.android.com/apk/res/android">
+    <item android:drawable="@android:color/white" />
+
+    <!-- You can insert your own image assets here -->
+    <!-- <item>
+        <bitmap
+            android:gravity="center"
+            android:src="@mipmap/launch_image" />
+    </item> -->
+</layer-list>

animations/android/app/src/main/res/values/styles.xml

@@ -0,0 +1,8 @@
+<?xml version="1.0" encoding="utf-8"?>
+<resources>
+    <style name="LaunchTheme" parent="@android:style/Theme.Black.NoTitleBar">
+        <!-- Show a splash screen on the activity. Automatically removed when
+             Flutter draws its first frame -->
+        <item name="android:windowBackground">@drawable/launch_background</item>
+    </style>
+</resources>