Fix activity lifecycle

Some renaming caused a lot of churn in this patch; the only substantive changes are:
  org/mozilla/search/MainActivity.java
  org/mozilla/search/tests/MainActivityTest.java

MainActivity:
  Before creating new fragments, or adding new fragments to the tree, explicitly check whether they've already been created.
MainActivityTest:
  Checks whether the proper fragments are visible after initial load, search, and having the activity recreated.