Fix onIsStashedChanged() not called in 3 button mode

Calling this ensures we update touchableRegion accordingly.

Since this will also start the stashed handle region sampling,
which we don't need for 3 button mode, also adjust logic to
check that the StashedHandleView is actually visible.

Flag: None
Test: Lock screen, run `adb shell cmd uimode night yes`, unlock
Test: Lock screen, rotate, unlock
  In both cases, verify touchableRegion is accurate
Fixes: 315393203
Change-Id: I951af6b97cad0ee0f362dc22f3996cfd9286a21e