Remove unnecessary onRefreshKeyboard call.

When using the sliding spacebar language switch on ICS, both
LatinIME.toggleLanguage() and
InputMethodService.onCurrentInputMethodSubtypeChanged() invoke
LatinIME.onRefreshKeyboard().  This change eliminates the first one if
it isn't necessary.

This change also cleans up the followings.
  * reuse theme Context when the keyboard theme hasn't been changed.
  * clear the Keyboard cache when theme has been switched.
  * eliminates unnecessary Context reference from LatinKeyboard and
    SlidingLocaleDrawable.
  * recycle Bitmap and reuse Canvas in KeyboardView.

Bug: 4725930
Change-Id: I87366e9304879d94d12b7345adea768d86d43519