Enable Text core tests

Some of the tests were disabled because of the access permissions. This
CL:
* Removes @Suppress annotations
* Annotates the test classes with related annotations such as @SmallTest
* Removes unused imports
* Replaces android.test.sitebuilder.annotation.SmallTest annotations with
  android.support.test.filters.SmallTest

Bug: 29879498
Change-Id: I9e6e326bc6eaf365d7d5c6d1f7edec35e6957302