Move text coretests to JUnit (1)

* moved android.text package core tests to AndroidJUnit
* replaced manual mocks with mockito
* removed redundant "throws Exception" in all touched
  files

Bug: 31223263
Test: adb shell am instrument -w -e package android.text\
      com.android.frameworks.coretests/android.support.test.runner\
      .AndroidJUnitRunner

Change-Id: I6ae08df24064720635c4acc0d9085a539a317ac0