[IME]: Make mFullscreenArea invisble when candidate/extract frame not used

The flicker happens when the app in landscape mode with focusing the
search editor without enabling full-screen editText, after typing something
and pressing search button, the app quickly clears the editor focus then
makes the main activity focused and starts the fallback connection to
enable the fullscreen mode, so that the extracted view been inflated and
occcupied in mFullscreenArea,

Even though the full-screen mode changed to false during hiding the
keyboard, in updateExtractFrameVisibility() still makes mFullscreenArea
visible and flicker until the keyboard is hidden.

As the mFullscreenArea contains 2 frames: ExtractFrame & CandidateFrame,
Since ExtractFrame is used when enabling the full-screen editor mode,
for non-full-screen mode, we can rely on the candidate frame visiblity
to see if mFullscreenArea has to set visible, in case flicker happening
by unnecessary visible setting.

Fix: 211085869
Test: manual as steps:
 1). Set device dark theme is on.
 2). Open Messages on Landscape mode > Split screen.
 3). Tap Search box > input something.
 4). Tap Search icon on keyboard.
 5). Expect no flicker happens when hiding the keyboard after 4).

Change-Id: I0d50ae68016993329e8bb84b90ec835445f3db31