Move examples to //sky/sdk/example

We're trying to follow the pattern described in https://www.dartlang.org/tools/pub/package-layout.html

TBR=iansf@google.com

Review URL: https://codereview.chromium.org/1210173004.

apk/stocks/org/domokit/stocks/StocksActivity.java

@@ -18,6 +18,6 @@ public class StocksActivity extends SkyActivity {
     @Override
     protected void onCreate(Bundle savedInstanceState) {
         super.onCreate(savedInstanceState);
-        loadUrl("https://domokit.github.io/sky/sdk/lib/example/stocks/index.sky");
+        loadUrl("https://domokit.github.io/sky/sdk/example/stocks/index.sky");
     }
 }

sdk/README.md

@@ -88,7 +88,7 @@ to the underlying render tree.
  * To learn how to run Sky on your device, please see the
    [Running a Sky application](#running-a-sky-application) section in this
    document.
- * To dive into examples, please see the [examples directory](lib/example).
+ * To dive into examples, please see the [examples directory](example/).
 
 Services
 --------
@@ -146,7 +146,7 @@ Sky applications inside the `SkyDemo.apk` harness.  The `sky_tool` script
 expects to be run from the root directory of your application pub package. To
 run one of the examples in this SDK, try:
 
-1. `cd sdk/lib/example/stocks`
+1. `cd example/stocks`
 
 2. `pub get` to set up a copy of the sky package in the app directory.