Move text coretests to JUnit (2)

* moves android.text.util, android.text.format and
android.text.method packages core tests to AndroidJUnit
* replaced manual mocks with mockito usage
* cleaned up dead code

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

Change-Id: I7b13c10521381e4f26ca8e6189392914238a21dd