docs: Fixed ViewPager animation tutorial. Bug: 12815243

Went through the tutorial and fixed a few minor issues. There is
confusion with Fragment and FragmentManager from the support library
vs. the main Android APIs. Clarified this by showing some imports
explicitly. Tested the new code from the page on Eclipse and it works
as expected.

Change-Id: I8494a72f4c760a03a6b8f3322c5754e315f22e7c
Review: http://quixote.mtv.corp.google.com:8002/training/animation/screen-slide.html