Fix broken Backspace/ForwardDelete tests

* correctly use @Before to intstantiate
* get rid of useless KeyListenerTestCase class
* move to KeyUtils, refactor and document util method to generate
  a KeyEvent

Bug: 37991689
Test: adb shell am instrument -w -e package android.text.method
com.android.frameworks.coretests/android.support.test.runner.AndroidJUnitRunner

Change-Id: Ibbce351f31eb62492cd1c7c920fdef44df89b683