Hide soft-keyboard when no editor is focused

In case seeing the unexpected fallback keyboard layout when focusing to
the window without an editor from the app has shown the IME.

Note that this behaviour change affects when the IME focus switching to
the next window with following configurations from an IME shown window:
  1) SOFT_INPUT_STATE_UNCHANGED state without an editor
  2) SOFT_INPUT_STATE_VISIBLE state without an editor
  3) SOFT_INPUT_STATE_ALWAYS_VISIBLE state without an editor

Also, we guarded this platform behavior with DeviceConfig platform flag
in input method manager namespace.

Fix: 240701729
Bug: 243897178
Test: manual as issue steps
  1) In Launcher, swiping up to enter all apps page.
  2) Tap the search bar to show IME
  3) Type "Keep" to show Keeps app's shortcut
  4) Tap Keeps app's shortcut
  5) Verify IME won't be shown on top of Keeps app's activity
     (As the app set SOFT_INPUT_STATE_UNCHANGED without focusing an
      editor by default)
Test: atest KeyboardVisbilityControltest#\
         ImeInvisible_softInputAdjustNothingFromImeShownApp
Change-Id: I0c502bec5c5e21dabde08d610a9a1969c1229b08