[Android] Crosswalk Sample code

This is the initial version of Crosswalk Sample app.
It uses Crosswalk Embedding API, and covers below API usage:
1. XWalkView UI Inflation
2. Multiple XWalkView instances
3. Manifest
4. XWalkResourceClient and XWalkUIClient
5. Preferences
6. Page Navigation
7. Pause/resume JavaScript Timers
8. Visibility Change

BUG=XWALK-1230