Ability to switch between prefix- and full-text search.

- Reorganized the Search fragment a bit, so that it's a permanent part of
  PageActivity, and pops up instantly on top of the current fragment (does
  not go on the backstack)
- The Search fragment now encapsulates two sub-fragments:
  TitleSearchFragment and FullSearchFragment.
- Not using any kind of ViewPager or tab component... just the two
  fragments overlaid in a FrameLayout, making the code somewhat simpler.
- The Search fragment also contains a blank layout where "recent" searches
  will go. (Recent searches appear when the search field is empty)
- Made the size of list items consistent between Title and Full search.
- Made the "Search" action bar item part of PageActivity, so that it
  appears globally, no matter what type of fragment is at the top of the
  backstack.

Change-Id: Iede993cbf6569cddc5f40d8f92a6d5a9a85c723f