Refactor SimpleKeyboard into SimpleKeyboardView

This modifies SimpleKeyboard to extend FrameLayout (and consequently
removes the now redundant input_view layout). Also removes the strong
dependency on SimpleInputMethodService by introducing KeyPressListener.
Renames some fields and methods to better reflect what they represent.
Modifies SimpleInputMethodService to only inflate the views once in
onCreateInputView, rather than re-inflate every time onStartInputView is
called. Lastly optimized KeyCodeConstats to have a static switch
statement rather than initialize a HashMap.

Flag: EXEMPT testfix
Bug: 394328311
Test: atest InputMethodServiceTest
Change-Id: I14c3ad5d7f800e421fd4377c9ded5bf3d0af232f