Make Spacer as extended Key class

java/res/values/attrs.xml

java/res/xml-sw600dp/kbd_phone.xml

java/res/xml-sw768dp/kbd_qwerty_row4.xml

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

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

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

...ndroid/inputmethod/keyboard/internal/KeyboardBuilder.java