Remove checks for Activity Context when creating SearchManager

Since https://android-git.corp.google.com/g/3880
all activities create a SearchManager object, to handle
saving and restoring the search dialog. This broke
ActivityUnitTestCase, since ApplicationContext.getSearchManager()
threw an exception in non-activity contexts.
This change removes the activity context check from
getSearchManager(). Since SearchManager is now just a thin
wrapper for SearchManagerService, there shouldn't be anything
activity-specific in it.

Fixes http://b/issue?id=1926254