Use String instaed of CharSequence in Key class

java/src/com/android/inputmethod/keyboard/Key.java

java/src/com/android/inputmethod/keyboard/Keyboard.java

java/src/com/android/inputmethod/keyboard/KeyboardView.java

.../com/android/inputmethod/keyboard/internal/KeyStyles.java

...ndroid/inputmethod/latin/suggestions/MoreSuggestions.java

...android/inputmethod/keyboard/internal/KeyStylesTests.java