Improvements to the SampleApp (primarily Android).

Improvements to the SampleApp (primarily Android).

Reviewed at http://codereview.appspot.com/4587042/

Android
- Added buttons for interaction without a keyboard.
- Added the ability to zoom in to a specific point (roughly).
- Added event handling (for showing a slideshow, for example).
- Allow changing screen orientation
- Updated README file, explaining how to build

Multiplatform changes
- Added SampleApp header file
- Remove FPS when turning off measure FPS mode

git-svn-id: http://skia.googlecode.com/svn/trunk@1596 2bbb7eff-a529-9590-31e7-b0007b416f81