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

Change-Id: Ic51a044901a63c4cec907163e35cd5e458f36dde

Change-Id: I75d4d1625e0925d01ae84c9577e15087d83e4191

Change-Id: Id3974642b62b012968c537eff7c1a7ee37e2def2

Previously the key preview was implemented by PopupWindow.  The key
preview popup window would be dismissed and shown each time the key
was released and pressed.  It turned out that it was taking several
milliseconds.

This change implements the key preview by a simple TextView which will
be layout-ed at absolute coordinates within the screen FrameLayout.
And dismissing and showing the key preview is implemented by the
TextView's visibility.

This implementation needs careful coordination of candidates.xml
layout and LatinIME.onComputeInsets to control suggestion strip
visibility.

Bug: 4179964
Change-Id: Id6347cb35b91eb14ab20dd2f312e58b54b6625a7

Change-Id: Ie473c9fced9f0a8da5643390b44aa4b86128396d

Change-Id: I8b9fe507885ef5ae7cb09db93a977e23b1bbd0db

Change-Id: If6b0180c63738e5e78a20fe5b938ef50d62865d6